Often what determines whether an object sinks or floats in water is its density. If the object is denser than water, and thus weighs more than the water it displaces when submerged, then it sinks. If it is less dense than water and therefore weighs less than the same amount of water, it floats.

However, the physical world is complex and there are other factors to consider in order to fully understand floating and sinking. Generally, all metals sink in water because they are denser than water, which means that a quantity of any metal weighs more than the same volume of water. Still, metal objects can be made to float on water. A ship, for example, floats as a result of its shape and the fact that its hollow hull is full of air. These factors reduce the density of the ship as a whole, so that it displaces a huge amount of water relative to its weight.

Some objects will float on water not as a result of human engineering, but because of a property of water called surface tension. Surface tension is a force that results from the attractive forces among the particles of a liquid. Water's relatively high surface tension is due to the particularly strong bonds that form between the hydrogen atoms in water molecules. These bonds create an attractive force among water molecules, causing them to pull toward each other.

This attractive force acts to minimize the surface area of a quantity of water. It also causes the water surface to resist any attempt to increase its surface area. When a small object is carefully placed on the water surface, gravity begins to pull the object down into the water. The resistance of the water's surface to this incursion and the expansion it would cause can, however, prevent the object from penetrating the surface, and cause it to continue to float in spite of gravity. Once the object penetrates the surface, however, it sinks because its density is greater than that of water.

Substances such as soaps and detergents interfere with the bonds between water molecules and greatly reduce surface tension. They do this by forming bonds of their own with individual water molecules, thus occupying hydrogen atoms that would otherwise be bonded to the hydrogen atoms of other water molecules.