A box is shown to your spectator(s).
You hand it to your spectators -- ask them to try to open this mysterious box.
They will find it is totally impossible to open because it is completely sealed.
Next, you borrow a bank note, bill, or any other small object.
You then place the box and object behind your back (or place it under a table, under a napkin, etc.) and, in the blink of an eye, the object in INSIDE the box!
You hand the box out once again. No matter how hard they try, they cannot open the box!
Very Easy to do -- once you know the secret. The Mexican Bill Box
-- lots of fun to perform!