Answer:
What coin doubles in value when half is deducted?
4 rubber ducks were floating in the bathtub, 2 floated away and 2 drowned. How many ducks are still alive?
What is the difference between yesterday and tomorrow?
What goes up and never comes down?
What always comes into a house through the keyhole?
I stand in one place yet fill the whole room. I can be filled with molten rock and come in every hue. What am I?