This is too good not to use in the classroom… mwa hahahaha. (Found via Digg: here)
A simplistic explanation (which is all I wanted) is here:
Your brain can make your hand write a 6 in the normal direction no problem at all, and it can circle your foot in a clockwise direction, no problem at all either. But when you try to do both types of movement at the same time, big problems.
This is because your brain now has to send very different movement instructions to your arm and leg together at the same time. Your brain is simply not used to doing different things with your arm and leg at the same time in this way.
Note: another one is to “try to simultaneously rotate the index fingers of both hands in the same direction (clockwise or anticlockwise). Do it slowly at first, then faster, and faster…. Pretty soon, they’re going in opposite directions.” (from metafilter)