Dogged drifter

Dogged drifter

 
What you need:

A ping-pong ball
A plastic cup
Water

Instructions

Fill the cup almost full and float the ping-pong ball on it – it should be impossible to stop it drifting to the rim.

Add more water until the cup almost overflows and the surface is raised. Now you should be able to keep the ball in the centre, though it looks unstable.

Results & explanation

The water’s surface tension pulls down on the ball but water is also attracted to it and climbs up its sides. Overall the pull is diagonally down.

Near the rim, the water between ball and rim flattens, so the overall pull is towards the rim. But in a bulging cup, the water’s surface is almost vertical at the rim, so the overall force pulls the ball to the centre.