Some objects float while others sink because of their density relative to the density of the water they are placed in. An object will float if its average density is less than that of water, meaning it is lighter than the amount of water it displaces. Conversely, an object will sink if its density is greater than that of water because it is heavier than the volume of water it displaces. This principle is explained by the buoyant force, which is the upward force exerted by the displaced water. When this buoyant force equals or exceeds the object's weight, the object floats; if it is less, the object sinks. The density of an object depends on how tightly packed its particles are; for example, wood is less dense than water due to its porous structure, so it floats, whereas metals are denser and usually sink unless shaped to trap air, like in a boat's hull. Objects with trapped air or low-density materials inside can also float even if their solid parts are denser than water. This is why some objects float while others sink.