Have you ever taken a cup out of the dishwasher, put it on the counter, and then it moved all by itself?

Question

This happens to me from time to time. Sometimes it's a plate, today it was a plastic, blue cup. Can someone please explain this phenomenon? I tried googling it but I didn't find anything relevant. Thanks.

Answer 1

I'm guessing that there's still a little bit of water on the plate/cup so it slides a little.

Answer 2

If it's still damp or wet, and has enough water on the edge / rim of the glass if upside down or bottom if right side up, or the bottom of the plate or rim if it's upside down, the water will allow them to slide, especially if you put them down and not hold them still.  As you put an item down, there's still enough force in it that, unless you hold it to make sure it doesn't move, will allow it to slide unless the surface won't allow it to.

Answer 3

i would guess that the plate or cup is still quite warm, even hot.

Also, there is probably still a bit of water on the cup/plate. That will form a tentative seal, because the water forms a light bond with both the table-top and the plate. It is similar to the way a miniscus forms, but instead of the sides of a glass, this now clings to both surfaces: the cup or plate, and the tabletop.

Then the heat of the cup or plate warms the entrapped air. That warmed air expands and increases the air pressure of the trapped, contained air, and the water's 'stickiness' creates a seal, so the pressure does not immediately leak out. This increased pressure pushes up on the inside of the plate or cup, and counters the force of gravity, lifting the object slightly, so the net force on the cup is nearly balanced, nearly zero - it is just floating on the thin layer of water around the meeting with the tabletop.

So now, the plate/cup is almost floating, and any slight shove or some slight slant in the table will set it off moving.

But a scientific explanation should include some way to test it. If you dry the cup/plate and the table-top really well, it cannot form this water-seal, and the plate should not float. Also if you wait until the dishes are cold, even if they are wet, they should not float. These two tests should provide reasonable proof that the explanation is correct.