Don't know if this has been posted before here, but this one is the best.
Woman twirling, which direction? Watch for awhile and she changes directions.\
I showed this at a presentation once. Everyone looked at the twirling figure like "so what?" I then had everyone stand up and twirl in the direction of the woman.
Immediatley, about half the room starting turning clockwise, and the other half started turning counter clockwise.
After the surprise and laughter ended, we all watched to see if/when the direction would change.
Hint, I find it helps while you're looking at it to blink rapidly a few times, to get it to change.