Water has adhesive and cohesive properties. Adhesion (the tendency of water to stick to itself) is the reason for surface tension. This is because the hydrogen bonds the water molecules form with each other keep the water together. At the top of the water, there are fewer molecules of water to bind to (no molecules above the top), and so they can bind more strongly with those to the side and beneath them. Because of surface tension, some insects can walk on water if they are light enough to not penetrate the top layer of water. Hope this helps! :)
