I can’t walk on water, but the water strider insect shown to the left, can and does – hence its name water strider or pond skater. The reason why the water striders can walk on water is simple, they are very small (the insect show above has a body maybe a cm long) and the surface tension of water is quite large.

So what is surface tension? Liquid water only exists because water molecules attract each other, with an energy of around 10^-20J or equivalently a force of around 10^-10N. It is this energy that holds liquid water together into drops, and prevents it evaporating. These attractive forces between the water molecules pulls them towards each other and puts the surface of a liquid under tension – the surface always wants to contract like a stretched elastic band – hence the name surface tension. The surface tension of water is around 70 mN/m, or 0.7 mN/cm.

If you look at the image of the water strider you can see it has 4 long legs, each as long as its body, which is about 1 cm. In each case all the lower part of the leg (what I guess you could call the shin) is in contact with the water – this is what causes the 4 elongated dimples in the surface of the water.

So for each leg there is a length of around 1 cm in contact with the water. Then as water’s surface tension is 0.7 mN/cm, a total length of 4 cm can support a maximum force of about 2 mN. At rest the force is just gravity, and taking the acceleration due to gravity as 10 m/s², 2 mN is the weight of 0.2 g. Water striders are about 0.01 g heavy, and so are easily light enough to be supported by the force of surface tension.

However, the force surface tension exerts increases only linearly with the size of the organism, and this is a problem as mass increases as the volume, which increases as the cube of the size. My feet are roughly 10 cm long, so the surface tension force is about 7 mN. I am about 75 kg which gives a weight of 750 N.

So, of course I can’t walk on water, I would go straight through the surface. But for things the size of a water strider or smaller, walking on water is as simple as a walk in the park.