What causes the surface tension of water?

Water molecules are dipolar - they have both positively charged and negatively charged regions due to differences in the electronegativity of atoms (this is how strongly they attract electrons). This property enables intermolecular hydrogen bonding between the slightly positive hydrogen and slightly negative oxygen atom of adjacent molecules. The bonds are weak, but have a strong combined effect as there are so many. The attraction of water molecules to each other is called cohesion and results in surface tension because molecules at the surface tend to 'stick' to neighbouring molecules, providing resistance to pressure from outside the body of water.