The weight of the insect and structure of their feet in combination with the surface tension of water allow some insects to walk on water. Water molecules are more attracted to each other than they are to other materials, so they generate a force to stay together called surface tension. If the force that the insect foot makes on the water, is less than the surface tension of the water, the bug will float. If the force of the foot is greater than the surface tension, the bug will sink. What determines the amount of force the foot generates is dependent on weight and foot shape. If the foot is flat and wide, the weight is dispersed across a greater surface area and the force is less than if the foot were sharp and thin.

Water doesn't behave the same way on small scales. It's the reason why water forms drops on a surface, something called surface tension. This allows water strider insects to not sink in and skim on the surface.

Small animals can run over water because of the surface tension water has. You can see how water tension works if you put a drop of water on a table. The molecules of water in that drop will stick together to create a dome shape, rather than lying flat and even on the tabletop.

Water molecules stick together and a force must be used to surface of the water. While this force seems small to us, very small animals that don't weight that much will not create enough force to separate the molecules. Small insects, spiders, and even a species of lizard, the basilisk lizard, are able to take advantage of the surface tension of water and can move over the water without sinking into it.