The Detachable Thumb (unknown originator) is a very simple close-up illusion.
The performer appears to remove the end of his own thumb; moving it back and forth along his hand or lifting it up.
There is no particular secret to this illusion: the performer simply holds out one hand sideways with the palm out and the tip of the thumb folded down behind the hand. Then he places his other thumb bent so that the tip appears to be connected to the folded-down thumb with the forefinger bent to cover the joint.
There is no known originator of this trick, and it is likely to be among the oldest tricks in the world.