There's a not-so-recent video by a user named quirkology here, where he shows bets that usually work. From @2:19, he puts a wine bottle cap at the center of a glass filled halfway with water, and this cap eventually moves to the sides.
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And the trick he shows to make the cap rest at the center without touching it is to add water until the glass if entirely filled, then the cap rests in the center.
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Does anyone have an explanation of that phenomenon?
This is because that the cork floats to the highest point it can.
The water is not flat in the glass - it curves up a the edges so the cork gets higher by going to the edge.
When the glass is full, really completely full just above the level of the rim of the glass, the water will be bowed a bit so that it goes down at the edges and is highest in the middle - so the cork goes to the middle.
NB the cork floats to the highest point it can because it displaces water by resting on the water and the higher it goes the lower the water in the glass can rest.
It has to do with water tension. When the glass is filled to the very top, the water causes a dome to form. The cork floats to the very top and this top is in the middle of the glass.
When the glass is half full, the water closer to the edge of the glass is at a higher level than in the middle. This causes the cork to float to the side.
