The greek philosopher-scientist Archimedes is famous for sitting down in the tub one day and then running naked through the streets of Athens shouting "Eureka!" What had he discovered?
It was the law of buoyancy, nowadays called the Archimedes' Principle. You can demonstrate it yourself without even running around naked.
Here's how. You'll need one of those small kitchen scales, You'll also need a pie pan, a glass, and something small that floats, like a small block of wood.
Weigh the block of wood on the scale. Then remove the wood and place the pie pan on the scale. Write down the weight of the pie pan, too. Now fill the glass entirely to the top with water and place it carefully inside the pie pan. Take the small wooden block and put it in the glass, too. This will cause some of the water to spill out of the glass and into the pie pan. Next, remove the glass altogether, being careful not to spill any more. Now you just have a pie pan with spilled water in it.
By subtracting the weight of the pie pan from the weight of the pan with spilled water in it, you get the weight of the spilled water alone. Guess what? If you were careful, the spilled water will weigh exactly as much as the block of wood!
This is Archimedes' Principle. An object will float if it displaces as much water as it weighs. This is true for any object, even huge, iron battleships!