Pond skaters can walk on water because water has a kind of skin on top. All liquids have a similar kind of skin on the surface. It is there because of the atractin forces between particles in the liquid. If we could see the particles in the liquid, they would look something like this:
The particles are attracted equally by particles on all sides. But the particles at the surface are only attracted from underneath. They feel more downwards force than upwards force. This makes the surface particles stick together and form the skin. This effect is called “Surface Tension”.