There are two crucial factors in determining whether an object will sink or float. The first is the relationship between its weight (a measure of the force of gravity pulling down on the object) and its volume (the amount of space the object takes up). The second factor is the composition of the object—what material(s) it is made from and its overall shape.

When a boat, for example, is placed in water, it effectively makes a hole in the water—displacing, or pushing away, water that would otherwise be filling the hole. If the boat weighs more than the water that had been filling that hole, the boat will sink. But if the boat weighs less than the amount of water it displaces, then the upward force of buoyancy created by the water surrounding the boat will be greater than the downward force of gravity, and the boat will float.

When a block of wood is placed in water, it does not sink entirely because the wood block is less dense than water—that is, it takes up more space for the same weight. It only sinks until the upward force on the block from the surrounding water equals the downward force of gravity, so the wood block remains only partially submerged.

A block of steel placed in water, by contrast, will sink to the bottom. Steel, which is made mostly of iron atoms, has a much greater density than water or wood, primarily because iron atoms are much heavier than the hydrogen, oxygen, and carbon atoms of the water and wood. The steel cannot displace its weight in water, which causes the block to become entirely submerged.

In this video, each of the three boat designs floats at first. However, the two boats made using paperboard tubes and open-ended straws and cups eventually absorb or are filled with water. Because the water-logged boats are heavier than the water beneath them, they sink.

The only boat left afloat is the one that uses the pie tin. Even though the tin is made of aluminum—a metal—it does not sink for the same reason that large steel-hulled ships do not sink. Steel ships are shaped in such a way that the weight of the boat is displaced before the boat is completely underwater. Because much of the interior of a boat is air, the combination of the steel and the air is lighter than the average density of water. Since very little of the boat sinks below the surface of the water before it has displaced the weight of the boat, the boat floats.

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