why does a coin sink in water

In the case of the Coin Drop activity, the penny is at rest sitting on top of the card. Watch it as long as you like but, on it's own, the penny will not move from that spot. When you flick the card out from under the penny, you allow gravity (an outside force) to act on it and drop it into the glass. Since the coin is quite small and relatively heavy, the water displaced by it weighs a lot less than the coin itself so it sinks. If you could take the metal and beat it out into