1. Take 2 corks (e.g, from wine bottle). Longish ones are better.
2. Hold them in each hand, with your thumb and fore-finger, as in picture.
3. Now, in one continuous motion, exchange the corks between the two hands so that they are held as in the second picture.
4. Try to go back and forth between these two positions!!!
HINT: First try to give only one cork to the other hand.
SOURCE: This trick came to us from Europe, perhaps by way of Utrecht (Mark Overmars?)