If someone throws a crowbar in the ocean, it sinks, but a ship, which has a weight many times that of a crowbar, manages to float in the water. This interesting phenomenon known as buoyancy is the reason that boats float. The properties of buoyancy were first described by the Greek mathematician Archimedes, in what we now call Archimedes' Principle. This principle states that any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
Archimedes is said to have discovered the principle in the bathtub, which makes sense, because a bathtub is a great place to research buoyancy and displacement. He discovered that when an object (such as a scientist) is placed into a fluid, the level of the fluid rises, because the object has displaced some of the liquid. Archimedes realized that any object placed in water will displace its own weight or volume in water, which ever comes first. This is called the weight-to-surface area ratio.
He realized that different materials which have different densities but equal volume will have a different buoyancy. A bowling ball will sink, while a balloon filled with air will float, even though they occupy the same volume in space. This is due to the greater density of the bowling ball. Unlike a balloon, a bowling ball weighs more than the weight of the water it displaces, so the ball has a low surface area-to-weight ratio. A boat is essentially a hollow shell filled with air, meaning that it has a large surface area-to-weight ratio. Therefore, the boat will still be well above the surface when it has displaced its equivalent weight in water.
Boats float because of their design. When a boat is heavily laden, it will settle lower in the water, because its surface area-to-weight ratio is different than when the boat is light. A boat will remain floating and stable provided it is not overloaded.
Once someone knows how boats float, he or she can imagine how it is that other objects float. From pieces of wood to ducks, an object's weight-to-surface area ratio dictates its buoyancy. This also explains why boats sink: if the hull of a boat is breached, it begins to take on water, which makes it denser, causing it to displace more water. If the boat takes on enough water, it will become too heavy to remain buoyant.