I always liked this gag, because kids always like it! Yes, it is slightly "insulting" to your volunteer, but I have never met a child yet who didn't think it was funny that his or her hands turned the towel from clean to a dirty mess. Kids like the visual, fast humor of it -- even the kid who is the butt of the joke.
The effect is simple, and to be used as part of any routine where a volunteer from the audience has to touch one of your prized magical props. Before letting the volunteer touch the prop, you say you want to wipe his hands clean. Show a yellow, micro-fiber towel on both sides, rub it vigorously on the volunteer's hands, and then when you show the towel again, it is covered with big black hand prints.
Silly, and not the world's biggest mystery, but a lot of fun. This version of the classic gag is made in a good quality micro-fiber that looks and feels like terry cloth, but is much lighter and thinner. Some versions of this gag are big and bulky (necessitated by the secret way the cloth is gimmicked). But this version is easy to handle and quite thin. Get this now and one more big laugh to your kid show.