Surface tension is caused by the attraction between liquid molecules along the surface of the liquid. Most of the molecules in a liquid have several neighboring molecules that they interact with, but the molecules along the surface have no neighboring molecules above them, which increases the strength of the surface molecules with each other.
Surface tension can cause the surface of a liquid in a small container to be rounded. It can also support materials that are denser than the liquid as a whole. For example, a needle or paperclip can float on the surface of a glass of water, but if the surface tension is broken, they will both sink to the bottom.