What is surface tension?

Surface tension is the energy, or work, required to increase the surface area of a liquid due to intermolecular forces.

Surface tension in a liquid is caused by the attractive force exerted upon the surface molecules by the molecules beneath. It tends to draw the surface molecules into the bulk of the liquid and makes the liquid assume a shape that has the smallest surface area.

Surface tension of water can cause things that are more dense than water to float. This allows organisms to walk on water.

An example is a water strider. It can run across the surface of water because its weight is distributed among its six legs. The force exerted by each leg is too small to overcome the surface tension.

Here is video showing how a paperclip can "float" on water - it's actually being held up by the hydrogen bonds formed between water molecules which give water its surface tension. Video from: Noel Pauller