Step 1: Beginning the Trick
Put a small cup under a thin towel; or something slippery and not transparent.
Step 2: The Secret
Slowly pull the cup under the towel to the edge of the table, letting the cup slide off the table into your lap. Gently hold the towel around the cup's base as you do so, which will make the leftover figure maintain shape as the cup slides out of the towel without letting the audience know.
Step 3: The Trick
All you have to do now is push the leftover figure down to the amazement of the crowd!
Step 4: Special Effects
For extra fun, stand up as you do step 3, so there will be a sound when the cup "goes through" the table! ;)