Repeat the experience. Large copper coin put a clear glass. Through its walls the coin is clearly visible. Pour in a glass of water - the coin will disappear. Of course, if you do not look into the glass from above. On the basis of this optical effect can come up with a number.
Now get ready for demonstration. Take a coin and pre-glue it to the outside of the bottom of the glass. Show the audience a glass of water. There's nothing in it. Not releasing the Cup from his hands, offer to cover it with a handkerchief. Lower the glass down and hold it so that the audience was now looking at him from above. Show everyone in the glass was the coin. We know that actually it is not glass, and outside.
Using such an optical property that can be done a number of similar and yet different from each other tricks. All depends on your imagination.