Buoyancy is the tendency for something to float. When something is on water it displaces an amount of water. Have you ever tried throwing a rock in water. If you did you most definitely know that it's will sink. However if you try putting a paper boat on water it floats. How come? Well, if you check how much the water displaced weighs and how much the paper boat weighs you would see they're almost equal. Which determines why the paper boat floats. If you weigh a rock though and how much the water displaced weighs, you would see the rock weighs more.