Why do water striders not sink?

Water striders are very light-weight insects. Their legs are covered with millions of tiny hairs that trap air bubbles. These air bubbles along with surface tension of the water keep water striders afloat. Water molecules attract one another very strongly; this is known as ‘cohesion’. The resultant force acts downwards on the surface of water, above which there is only air and no water, so a flimsy solid layer is formed. This effect is known as surface tension. Because of surface tension water always ‘tries’ to keep the smallest possible surface area. Thus, raindrops on the window pane are spherical.