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Key Facts In This Video
If you drop a styrofoam ball in the center of a glass of water, it immediately tries to travel toward the edge. 00:22
Water molecules attract each other more than they attract air molecules. This causes surface tension, an effect that makes water behave as if the surface is covered by a membrane. 00:42
In a completely full glass, the surface is convex, so the balls move toward the highest point -- the center. In a partially filled glass, the concave surface's highest points are at the edges, which is why the balls move there. 01:45