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.
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.
Does anyone have an explanation of that phenomenon?