Just imagine the situation. You are about to fix a magical prop with the mallet supplied. You give a few strokes and then hand the mallet to an assistant spectator to do the same. When she tries to strike, all of a sudden the head falls off!
Acting surprised, you assemble the head and ask her to pull it to see whether it is properly fixed. When confirmed, you hand the mallet over to her once again and request her to strike the prop. As she attempts to do so, the head again falls off.
This is an excellent way to get a good laugh out of your audience. This type of comedy gets better with repetition. However, please don't overdo it!