As with most liquids, water molecules are normally attracted to each other. This attraction creates tension at the surface of the water, often referred to as a thin "skin," which allows some insects to glide across it. When leaves, twigs or other organic substances fall into water and begin decaying, they release compounds known as surfactants. This interaction breaks the surface tension, which in turn allows air to more easily mix with water and creates bubbles. These bubbles congregate as natural foam. However, not all foam is natural. Certain man-made products, including detergents, can cause foam that is similar in appearance, but may be harmful to fish and other aquatic life.