To float a cork at the center of a glass of water.
Take a glass tumbler and fill it with enough water to reach almost to the brim and then drop a small cork into the glass, try to make the cork float in the centre of the water without touching the sides of the glass. The cork always drifts to one side. Fill more water to the glass, pouring carefully from another glass until the water rises slightly above the brim. Due to surface tension, the water will form a convex surface and the cork moves to the centre where the water level is highest and floats there.