What causes surface tension in water?

Surface tension in liquids is due to a phenomena called cohesion. Cohesion is in turn the result of intermolecular forces (IMFs) acting between molecules of the liquid. Water experiences cohesion because it has dipoles; these dipoles cause dipole moments and Van der Waals bonds. For more information on Van der Waals bonds see: http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/waal.html#c1

from wikipedia: Surface tension is caused by the attraction between the molecules of the liquid, due to various intermolecular forces. In the bulk of the liquid each molecule is pulled equally in all directions by neighbouring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid, but there are no liquid molecules on the outside to balance these forces, so the surface molecules are subject to an inward force of molecular attraction which is balanced by the resistance of the liquid to compression.