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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.

$\qquad\qquad$ like this

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.

$\qquad\qquad$ just like this

Does anyone have an explanation of that phenomenon?

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    $\begingroup$ Possible duplicates: physics.stackexchange.com/q/71292/2451 and links therein. $\endgroup$ – Qmechanic Sep 16 '15 at 23:49
  • $\begingroup$ look at the glass from the side. in one case, the water should be concave, in the other case, convex $\endgroup$ – njzk2 Sep 17 '15 at 3:39
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    $\begingroup$ @Qmechanic - I can see your point about the duplicate, but the duplicate question does not address the (interesting) case of the full glass and cork moving to the middle of the water. $\endgroup$ – tom Sep 17 '15 at 8:38
  • $\begingroup$ @tom i 100% agree with you $\endgroup$ – user153330 Sep 18 '15 at 20:24
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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.

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  • $\begingroup$ thanks for your answer, but why does the cork always seek the highest level? $\endgroup$ – user153330 Sep 17 '15 at 8:21
  • $\begingroup$ @user153330 Archimedes principle and gravity are the cause.... It is the same reason that ice floats as high as possible in a glass of water and does not sink to the bottom - I will edit answer. $\endgroup$ – tom Sep 17 '15 at 8:42
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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.

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