Surface tension is is sometimes referred to as the skin on the surface of a liquid. However, technically, no skin forms at all. This phenomenon is caused by cohesion between molecules at the surface of the liquid. Because these molecules don't have similar molecules above them to form cohesive bonds with, they form stronger bonds with those around and below them. The result of this strong cohesion is the film-like membrane known as surface tension, which can allow small objects -- such as pine needles -- to float on top of them.

One characteristic of surface tension is that an object will encounter more resistance while passing through the surface membrane of a liquid than through the bulk of the liquid. Liquids with high surface tension exhibit significant resistance to penetration compared to the resistance experienced in the bulk of the liquid. Liquids with low surface tension, however, have less of a difference between the tension on the surface and in the rest of the liquid. Pure water, for example, has significantly high surface tension. If you place a small needle on the surface of pure water, the needle will float despite being more dense with water. However, if you mix soap with water, the surface tension lowers significantly, and the needle will sink. The soap has caused the tension level to drop closer to the level of resistance found in the bulk of the water.