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I think everyone should have seen a bowl of hot water moving by itself on a flat surface such as glass(seems like it is moving by itself but maybe there are some external force applied to it when it moves). What is the explanation of this phenomenon?

Hypotheses :

  1. Since the bowl is heavy, for it to move the friction between the bowl and the surface must be very very low. I have heard that water may form a thin layer between the bowl and the surface, but l'm not sure whether it is the case or not.

  2. I think the bowl can move by itself because heat makes the air below the bowl expand.

Edit : I will re-experiment those again and update the question in few days. Maybe I will take a video clip too. and I am not talking about a magic trick whatsoever.

My bowl looks something like this

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  • $\begingroup$ what direction does it move? How much does it move? Does it always move? Is it always a glass table? $\endgroup$ – Jim Dec 1 '14 at 16:51
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    $\begingroup$ Is there any moisture / water on the surface of the table under the bowl? If after it starts sliding you dry off the table and the bottom of the bowl does it still slide? $\endgroup$ – Brandon Enright Dec 1 '14 at 16:54
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    $\begingroup$ Or your table is slanted... $\endgroup$ – user10851 Dec 1 '14 at 16:55
  • $\begingroup$ Are you thinking of the Leidenfrost effect? Video: youtu.be/zzKgnNGqxMw?t=43s $\endgroup$ – iantresman Dec 1 '14 at 18:45
  • $\begingroup$ I do observe this phenomenon from time to time with plates and hot food on it. For me usually the plate also rotates in addition to mere displacement. The displacement is of no more then 2-3cm I recon and the rotation of the edge is of the same magnitude. The whole movement takes around the second, then the plate stands still. When I pick up the plate there is a sizeable amount of steam condensate on the underside of the plate and on the surface of the table. $\endgroup$ – Yrogirg Dec 3 '14 at 13:19
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The effect is real.

The heat in the bowl causes the production of steam in the cavity between it and the table. Depending on the temperature, this can be a far more powerful effect than mere thermal expansion of the air. The liquid between the bowl edge and the table acts like a liquid seal for a reasonably smooth and planar table/bowl interface - liquid being held in place by surface tension.

All the above give you a "hovercraft" which has extremely low friction. Now all you need it a tiny push. This could be a small slant on the table, but the expulsion of small amounts of steam & water from underneath the bowl (in random directions) can be sufficient to push - and that will generate random motion.

I think what you observed is real - all it needs is a well fitting hot bowl and a wet table.

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