I'm sure this trick is as old as time. It scares the kids for about four seconds, and then fascinates them. They want to figure it out. They want to be able to detatch their thumbs, willy-nilly, and then put them back JUST LIKE THAT.
There was a sticker on a wardrobe in my friends' garage, when we were growing up. In fact there were quite a few of these stickers on that wardrobe: a picture of a hand missing a finger, and the caption: Fingers Don't Grow Back.
But this nifty magic-trick suggests they do. Thumbs come off without pain and reattach without stiches and therapy. <span 1.3;"="">We entertain the kids. We wow them with our choice skills. We trivialise serious medical emergencies. All in a day's work.