Answer:
What always comes into a house through the keyhole?
What can you fill a room with that takes up no space?
An archaeologist unearths a coin bearing the inscription "57BC". How does he instantly know that it's a forgery?
When you put this in a heavy wooden box the box will become lighter
What coin doubles in value when half is deducted?
I only exist when you are here. Where you never were I can never be.