In one hand you hold an empty wine glass. And in the other–a small lock and a key that opens it. Give the key to a volunteer for safe keeping. Close the lock and let the audience verify that it is indeed locked shut.
When the lock is returned to you, place it in the bottom of the glass.
Then take a handkerchief and cover the wine glass.
Say some magic words and yank the handkerchief away from the wine glass. At the same time, the sound of the lock hitting the glass is heard by everyone. What happened? You show your audience that there is no lock at the bottom of the glass. Where did it go?
The lock is fastened on the stem of the wine glass! Did the lock fall through the glass?
The secret is that from the start you have two identical locks. And in the center of the handkerchief you have attached a small magnet.
Before performing the magic trick, attach one lock to the stem of the wine glass.
And then simply cover it with your hand. During the magic trick, cover the wine glass with the handkerchief in such a way so the second lock sticks to the magnet. Then simultaneously remove the handkerchief and let go of the first lock, which falls to the bottom of the stem and makes the sound everyone hears–giving the impression that the lock fell through the bottom of the wine glass.