Surface tension is the tendency of liquids to reduce their exposed surface to the smallest possible area. A drop of water, for example, tends to assume the shape of a sphere. The phenomenon is attributed to cohesion, the attractive forces acting between the molecules of the liquid (see adhesion and cohesion). The molecules within the liquid are attracted equally from all sides, but those near the surface experience unequal attractions and thus are drawn toward the center of the liquid mass by this net force. The surface then appears to act like an extremely thin membrane, and the small volume of water that makes up a drop assumes the shape of a sphere, held constant when an equilibrium between the internal pressure and that due to surface tension is reached.

A drop of water is small, but it is made of even smaller parts called molecules. Water molecules have bonds that hold them together. At the surface of the water, the molecules hold on to each other even more tightly because there are no molecules pulling on them from the air above. As the molecules on the surface stick together, they form an invisible "skin" called surface tension.

Because of surface tension, various small insects are able to skate across the surface of a pond, objects of greater density than water can be made to float, and molten lead when dropped into a cool liquid forms suddenly into shot. Water striders and other insects can walk on water without sinking. The surface tension is strong enough to hold them. The insects' feet make dents in the surface tension, but it doesn't break.

Water's surface tension (the ability of a substance to stick to itself) makes it an excellent substance to float heavy objects upon. Water not only sticks to itself, but also to other surfaces, and this allows it to move against gravity, which is very important to plants when transporting water form the soil to their leaves. This upward motion is known as capillarity or capillary movement.