You cannot do two things at once

[andrewducker]

While sitting at your desk make clockwise circles with your right foot.

While doing this, draw the number "6" in the air with your right hand.

Your foot will change direction.

Comments

[drainboy.livejournal.com]

I find it easy to do (if not trivial) unless it's my left foot and left hand.
Then it's kinda harder, but not impossible.