OK - watch this - from Discover Magazine.
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More on how it works, from New Scientist:
A gravity-defying illusion has won the 2010 Best Illusion of the Year Contest, held yesterday in Naples, Florida.
The visual trick involves a 3D construction of four slopes that appear to extend downwards away from a common centre (see video). When wooden balls are placed on the slopes, however, they bizarrely roll upwards as if a magnet is pulling them.
But the "Impossible Motion" illusion, created by Kokichi Sugihara of the Meiji Institute for Advanced Study of Mathematical Sciences in Kawasaki, Japan, is soon dispelled when it's viewed from a different perspective – each slope is actually sloping downwards towards a common centre.
We're fooled because we make the assumption that each supporting column of the object is vertical, and that the longest column in the centre is the highest. But in reality, the columns and slopes are angled to create the illusion.