As children we all played with paper boats in the tub. I always wondered why the boat floats while the pebbles I threw would always sink. When an object enters water, two forces act upon it. There's a downward force (gravity) that's determined by the object's weight. There's also an upward force (buoyancy) that's determined by the weight of the water displaced by the object. An object will float if the gravitational (downward) force is less than the buoyancy (upward) force.