# Physics of every-day life: rotating bag of tea

Whilst studying for my physics courses, I like to drink tea. Today, I noticed that if you pull a bag of tea out of a hot cup of water, it gradually starts to rotate, picking up speed as time progresses and reaching a asymptotic speed after a while. My first question is: why does it start rotating? Of course it has something to do with the liquid water entering the gas phase just above the cup of tea, but how does that give the bag of tea a directed motion to one side? I presume that the momentum of the gaseous water is quite randomly directed vertically, so I don't see a reason for a horizontal rotation. Second question, how is the direction determined? I haven't tried, but might the direction of rotation be equally probable to be left-handed and right-handed? Third question: might the shape of a tea bag have an influence on what we observe? If we take a pyramid tea bag, for example, does the behavior change? I will perform some of these experiments while making my cup of teas and report back to you :)

• I think it's just the torque that's stored in the thread... May 18 '13 at 12:35
• @Juris, I thought that as well, maybe the water makes the thread more flexibel, llowing it to unwind more easily. May 18 '13 at 12:51
• I would like to suggest that the increased mass of the wet tea bag is an important factor in unwinding the thread of the cord.
– user62451
Oct 19 '14 at 9:33
• If you think it's the steam over the hot tea, try holding the bag elsewhere. This is easy to check! Oct 19 '14 at 9:48

Of course it has something to do with the liquid water entering the gas phase just above the cup of tea, but how does that give the bag of tea a directed motion to one side?

Nope.

The teabag is dangled by a string. Remember that the string is made of wound up threads:

Now, the threads stay wound up because they fit well and they have a knack of being permanently deformed if held in the same orientation for too long (for example, if you loop a thread and press it for a while, the loop stays). Wetting them undoes this permanent deformation.

When you dip it in tea, however, the liquid is absorbed by the threads, and they expand. There is no longer enough room for them to stay tightly wound. Besides this, they lose the permanent deformation, so they have the motive and the means to unwind -- which is what happens.

• Unexpected but beautiful answer! So if we dip the tea bag into the water, without wetting the thread, it will not rotate? I will try it! May 18 '13 at 13:01
• @Erik: Well, depends, because water way rise up the bag and into the thread. (Plus the steam may be hot enough to loosen the thread). But the general idea is correct :) May 18 '13 at 13:02

I think the above is partially right, but not completely. After stumbling on this while prepping a cup of tea, I did some experimenting and found that this happens repeatedly with the same bag being dipped repeatedly. The thing that first caught my attention was when I dipped the bag back in the tea after having removed it the first time, the bag which had come to rotational rest in the air rotated in the opposite direction when dipped back in the tea, almost the exact amount of full rotations. My conclusion is that the increased weight of the wet tea bag causes the fibres to unwind as mentioned above (not the wetness of the string however) and when the bag is floated again in the water, releasing the string from the weight pulling down on it, the string rotates back to it's natural state of rest.