The Principle of Buoyancy
If a body which is lighter than a fluid is placed in the fluid, it will be immersed to such an extent that a volume of fluid which is equal to the volume of the body immersed has the same weight as the whole body.
For Example:
Place a 1'x 1'x 1' block of wood (with a density of 40 lbs./cu.ft.) in a container of water.
The density of fresh water is 62.4 lbs/cu.ft.
The block of wood will sink down into the water by a depth (D) such that the volume of water displaced by the wood equals the total weight of the block of wood.