Lets consider a container filled with water, where at least a few molecules are away from the surface. Every molecule is engaged in a tug of war with its neighbors on every side.

For every “up” pull there is a “down” pull, and for every “left” pull there is a “right” pull, and so on, so that any given molecule feels no net force at all.

At the surface things are different. There is no up pull for every down pull, since of course there is no liquid above the surface; thus the surface molecules tend to be pulled back into the liquid. It takes work to pull a molecule up to the surface. If the surface is stretched – as when you blow up a bubble – it becomes larger in area, and more molecules are dragged from within the liquid to become part of this increased area. This “stretchy skin” effect is called surface tension.

Surface tension plays an important role in the way liquids behave. If you fill a glass with water, you will be able to add water above the rim of the glass because of surface tension.

We can float a paper clip on the surface of a glass of water. Before you try this you should know that it helps if the paper clip is a little greasy so the water doesn't stick to it (rub it on your nose or forehead.) Place the paper clip on a fork and lower it slowly into the water. The paper clip is supported by the surface–tension skin of the water.

The water strider is an insect that hunts its prey on the surface of still water; it has widely spaced feet rather like the pads of a lunar lander. The skin–like surface of the water is depressed under the water strider's feet.