Position your left hand with your thumb is facing up.
Bend the first finger of your right hand over your right thumb.
Hide the upper half of your left thumb by folding it down and below your left palm.
Put your hands together so it appears the first finger on your right hand is wrapped around your left thumb.
Act as if you're pulling very hard and slide your right hand away from your left, being careful not to go too far.
Slide your right thumb back against your left hand and act as if you're reattaching it to complete this magic trick. You can pretend to use pressure to force it into position, or screw it back in place.