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Hammering a nail into a ceramic cup causes it to shatter when dry but not when submerged in water. Is there an intuitive explanation for this? And does it have anything to do with the Rehbinder effect?

Image taken from this video. A ton of similar videos (1,2,3,4,5) claiming to show the "Rehbinder effect" went viral several months before.


For soft wettable materials like paper, immersing in water decreases its stiffness, by untangling the fibers holding it together. But ceramics? Aren't the bonds too strong to get weakened by the action of water? I feel like this effect has more to do with the propagation of vibrations on the cup rather than its material properties, which might explain why the medium plays a role.

Merry Christmas!

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    $\begingroup$ On a related note, I've read that you can cut a (thin) pane of glass with scissors, if you do it underwater. But I've never tried it myself. $\endgroup$
    – PM 2Ring
    Commented Dec 25, 2023 at 8:10
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    $\begingroup$ youtube.com/watch?v=tEAxhMECluM $\endgroup$
    – anna v
    Commented Dec 25, 2023 at 8:54
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    $\begingroup$ This isn't to say the phenomenon is not real, but in the linked video is not strong evidence of it's reality. There is a jump cut between when the bottom of the underwater mug is occluded by the hand, and when the hammering begins, so the video could easily have been faked by drilling the hole carefully with a drill bit designed for ceramic. $\endgroup$
    – Vaelus
    Commented Dec 25, 2023 at 14:22
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    $\begingroup$ @AlphaLife with the exception of the last one, all your linked videos have the same cut between the nail being placed and the hammer being struck, so they strike me more as evidence of a 'meme' among people who like to make fake videos, rather than anything else. (In the last one the bottom of the cup is blown out, which would make it easier to remove the hole in video editing software. Or it might be real, I really don't know.) $\endgroup$
    – N. Virgo
    Commented Dec 27, 2023 at 13:47
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    $\begingroup$ I have a dollar, a nearby second-hand store, and a hammer. Is it supposed to be a particular kind of cup? $\endgroup$
    – g s
    Commented Dec 28, 2023 at 2:40

7 Answers 7

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First some background.

Glass and ceramics are stiff, but not totally rigid. Thin sheets are surprisingly flexible. To break a sheet of glass along a straight line, you scratch a line along the surface. You may wet the scratch, but cutting oil is often used. Then you bend the glass to pull apart the two sides of the scratch.

Once a crack is started, it is much easier to make the crack deeper and longer than it is to start the crack. The crack follows the scratch because there is an existing small crack that is easy to make bigger.

Stained glass artists often cut curves into glass. The same technique is used, but you have be be careful how you bend the glass. Bending tends to make a plane become a cylinder. The stress pulls the edges apart in a line. The crack may propagate along the line of stress, not the crack. It is possible to make the crack follow a gentle curve, but harder to make it follow a sharp curve. Working near the edge of glass may modify the cylindrical shape and allow sharper curves. Textured glass with varying thickness can be harder. Here is a video showing tools and techniques.

Is It Actually Possible To Cut Glass With Scissors Underwater? As the video Anna linked shows, water helps because it is a polar molecule. It exerts large forces at the end of the tip, helping it to grow. You may have noticed that a crack in a windshield might be stable until it rains.

You can do something similar without water. Loosely grip a corner of glass with a pair of pliers. Rotate the pliers to scrape one jaw over the edge of the glass. The rough face of the jaw hits local areas with enough pressure to fracture the glass. The angle nearly parallel to the edge keeps the stress from pointing to the center. The pliers pulls small pieces of glass out of the sheet.

Glass artists often do something similar with a diamond grinder or saw. This is always done in water. It is much more controllable that manual techniques.

Note that breaking glass is a good way to get cut. The videos offer some safety tips.


You may have seen a bullet hole in a window. A hammer blow to the same window might shatter the whole window. But the bullet leaves a cone shaped fracture.

A bullet hits hard enough that inertia becomes important. Glass in front of the bullet is fractured and pushed out of the way. Glass to the side begins to accelerate. But the fracture very quickly separates it from the shattering glass, leaving it unbroken.

A water jet works the same way. But the bullets are smaller and faster. This video explains a bit about them on the way to Six questionably legal pencil sharpeners. It is not the way I would go about it, but he makes more fun stuff than I do.


How does this apply to the cup? In air, the nail isn't a bullet. It puts a lot of pressure on a point, hard enough to make a scratch. Hard enough for the crack to propagate. But it also bends the bottom of the cup. The crack follows grows in a line out to the edge.

In water, two things are different. He hits the nail harder. And the water under the cup adds inertial forces that support the glass.


Update

I no longer think the explanation above is correct. It was based on the first video above.

Looking at videos 1 - 5, I see nails successfully being tapped relatively gently through glass underwater. I see nails being hit hard in air, and the cup still breaking. So inertial forces from hitting hard are not the full story.

Video 2 shows bubbles escaping when the hole is punched through. Inertial forces from being supported by water are not the full story.

One constant is that a hole is punched through the bottom of a cup. The sides of the cup form a strong rim. The bottom is not going to bend much. I doubt you could punch a hole in a flat sheet this way.

This Physics Girl video shows cavitation breaking glass. Fact-Checking this Viral Bottle Trick I doubt this is going on here, but it shows that unexpected things can happen.

Relevant - Why does diamond have lower tensile strength than Iron?

Bottom line - I don't know how it works.

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Not an answer, but I did the experiment and it works (mostly) and you can't put photos in comments.

The nail drove a neat looking hole in the cup. enter image description here

Turning the cup over revealed that the interior had spalled. enter image description here

Fragments of broken ceramic from the cup. enter image description here

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    $\begingroup$ Hope it will repair OK if that is your favorite coffee mug! The spalling on the inside looks authentic, so I will I take that as proof that the myth is true. $\endgroup$
    – KDP
    Commented May 8 at 23:27
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I'm 99% sure the answer is that it is not possible to hammer a hole into a ceramic cup using a nail, regardless of whether the cup is underwater or not.

This is not just because a ceramic cup is brittle, it's because it's very non-malleable. (Which of course is a related property.) When you hammer a nail into wood, the wood gets pushed to either side of the nail, forming a hole. It's pretty obvious that this is not going to happen if you hammer a nail into a ceramic cup. So if the nail is going to make a nice round hole then where is the material going to go? I don't think there's anywhere it can go, so the only thing that can happen is that the material will be forced apart as you drive the nail in and the cup will break.

Although: after writing this answer, user g s posted another answer showing that after performing the experiment, there was a neat round hole in the top of the cup but a big crater messy on the inside of the cup, since a chunk of ceramic had broken off and fallen down, rather than being pushed to the side. So it seems it is actually possible - but it results in a less neat hole than the videos appear to show, so I still think they are fake, as I describe below.

But wait, you posted multiple videos, isn't that strong evidence that it is possible?

Well, no. The first four videos you posted show fairly obvious signs of being fake. In all of them there is a conspicuous cut in the video, in between the cup being placed in the water and the hammer being struck. In addition, in each of them there's something covering up the hole, so you can't actually see the nail going in. So I think those videos are made by this method: (i) put a normal cup into the water; (ii) stop filming and replace it with a cup that already has a hole in, made using a drill; (iii) cover the hole with your fingers and pretend to hammer a nail in; (iv) edit it together so it looks like the same cup. This is an easy trick that can be done by anyone who owns a camera and a tripod.

The last video isn't done that way, as there's no cut and he's not covering up the hole. But I suspect there is already a hole in the cup and it's just been removed in video editing software. Everything is blown out and blurry, which might be to cover up the editing. Or maybe this one is actually real and it actually made a messy hole like in g s's answer, which we don't really get to see.

But isn't multiple videos evidence? Surely they wouldn't all be fake?

Again no. It's actually pretty common for a certain type of fake physics video to get lots of imitators. Video creators make money from views (or hope to get popular enough to do so), so when something goes viral there's a good incentive to imitate it.

A good example is the idea that you can make a vortex in water using batteries. You'll find lots of videos demonstrating that. I can't seem to find a very good debunking of that idea, but hopefully it should be obvious that it's not possible to do that, and that the videos are made by first stirring the water and then playing the video backwards.

Ok, but if it's fake then why do they mention the Rehbinder effect?

This is just part of a magician's patter. It sounds scientific, and you can even look it up and find that it's a real term, so it makes the fake videos seem more plausible. It can't possibly have anything to do with what's happening in these videos, because there are no surfactants involved.

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  • $\begingroup$ The nail could push the material out the other side. In this case presumably the other side of the hole would be much less "clean" looking. If any of the videos are real, this may be why they only show one side of the punched hole. $\endgroup$
    – Vaelus
    Commented Dec 28, 2023 at 2:31
  • $\begingroup$ Seems like Valeus got it right. Spalled out on the inside, neat little circle on the outside, but still definitely a cup. $\endgroup$
    – g s
    Commented May 8 at 23:59
  • $\begingroup$ @gs there's no evidence for anything like that in any of the videos, but on the other hand there's a lot of evidence for them being fake videos in the first place. If you try this and can show me a photo of the hole, then we can talk. $\endgroup$
    – N. Virgo
    Commented May 9 at 0:02
  • $\begingroup$ I did! physics.stackexchange.com/a/813636/285671 $\endgroup$
    – g s
    Commented May 9 at 0:33
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    $\begingroup$ Late update - I had an old coffee cup that I wanted to turn into a flower pot. So I tried punching a hole like the videos show. It worked. I used a steel punch with a flat end and a big hammer. I got a shatter cone like a bullet would produce, but with a much flatter angle. I also tried it with some glass jars. It didn't work. They seemed to be stronger. They broke into pieces. $\endgroup$
    – mmesser314
    Commented Jun 2 at 4:33
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EDIT: I should have watched the video first. Scroll to the end for additional remark.


Supposition:
For a ceramic bowl to fracture the fracturing must start at the rim.

The center of the bowl is a domed shape. A domed shape has high resilience against deformation.

Supposition:
The energy of a hammer blow (delivered along the length of a nail to the center of the bowl) propagates out to the rim.

Supposition:
As impact energy propagates to the rim the amplitude of deformation increases, as the rim is not as resilient.

With a sufficiently large deformation of the rim the material is deformed beyond the range of elastic deformation, and the bowl fractures.

Supposition:
When immersed in water the inertia of the water suppresses the amplitude of deformation as it travels from the center to the rim.

With the bowl remaining intact the nail is only pulverizing the ceramic that it is directly impacting, thus punching a hole through the ceramic.

EDIT:
In the video the fracturing of the cup does not start at the rim. So the rim supposition is not relevant here.
What remains is the suppression of deformation due to the inertia of the water. The motion of the deformation is extremely fast, which means that the opposing force arising from the inertia of the water will be extremely strong.

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Water has a lot of inertia compared to air. Because of this, the nail will push through the ceramic before it can move the whole bottom of the cup including the water behind it. Filling the cup with sand would probably also work.

Aren't the bonds too strong to get weakened by the action of water?

They are.

I feel like this effect has more to do with the propagation of vibrations on the cup rather than its material properties, which might explain why the medium plays a role.

No it doesn't, but it's a better idea.

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  • $\begingroup$ " Filling the cup with sand would probably also work. " I was thinking the same thing. The mug might also need sand packed around it for this to work since there will be forces pushing outward that could crack the mug. $\endgroup$
    – MacGuffin
    Commented May 9 at 7:43
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I think there's a reason why they all are hammering a nail through a cup bottom, and not, say, through a flat ceramic dish or through a sheet of glass.

The reason might be the sides of the cup. Note that when the cup is shattered in the air, the nail splits the bottom shifting pieces of the cup sideways. The water covering the tube part of the cup, being reasonably incompressible, provides inertia sufficient to hold the sides (and hence the bottom) in radial direction. Large surface and thickness of cup walls helps. So for the nail, the path of least resistance is chipping a cone in the bottom of the cup.

This hypothesis can be proven wrong by an experiment where a nail is successfully driven through a more-or-less flat dish with no significant vertical walls.

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This is how I would make a good fake of the effect without cutting the video and replacing the cup with a pre-drilled one. I would pre-drill a hole in the base from the inside with a ceramic drill, to just over 3/4 of the thickness of the base. If the outside is pierced this can be hidden with matching colour clay like material. Now it would be easy to punch a hole through with a nail without splitting the cup. Just making the point that it would be easy to fake a video of the effect even without switching or using post processing video effects. What I would like to see in a video to convince me it was not fake, would be to see several holes punched in random locations in the base and then break or cut the the cup afterwards, so that we can see cross sections of the holes and see if holes look drilled with parallel sides. I would also like to see the punched out plugs, which should match the punched out cavities and cavities should presumably be more hemispherical than cylindrical. If the effect does work as advertised it might work with glass cups and it would be a bit harder to fake punching a hole in a glass with a nail underwater.

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