The polarity of H2O causes the H2O molecules to "stick" (more or less) to each other. It also has adhesive properties and you can see that if you suck water up into a glass tube. The tube needs to be about 1/2 inch in diameter. You will see the edges of the water HIGHER than the middle. The edges are an example of the adhesive property of water and it forms a curved surface (a little like a U). The middle of the U (the lower part) is an example of the cohesive property of H2O.