Water skin

Water molecules want to cling to each other. At the surface, however, there are fewer water molecules to cling to since there is air above. This results in a stronger bond between those molecules that actually do come in contact with one another, and a layer of strongly bonded water. This surface layer creates a considerable barrier (surface tension) between the atmosphere and the water. Water has one the greatest surface tension ('skin') of any liquid.

Examples of surface tension:

Walking on water: Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

Floating a needle: A carefully placed small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.