Do you think it's easy to place a coin on top of a lemon that is floating in a glass of water? If you said yes, you're probably right.
This science experiment video tutorial will show you how to do a gravity experiment with a lemon and penny. It's easy to balance a penny on top of a lemon when it's on a table, but put that lemon in water and it's almost impossible. It will keep sliding off from the turing of the lemon.
Try cutting a lit into the lemon and wedging the coin inside, then put it into the water. The coin will remain in the bottom, always. When you stick the coin into the lemon, it turns over o get into a more stable position, because it should get its center of gravity. The coin is more dense than the lemon. Putting the coin on top cause the center of gravity to be higher than with the coin lower down.