A Paper clip float on water because of surface tension. If you place a paper clip on the surface of still water, it will not sink because of surface tension. In liquids, molecules are attracted to each other in all directions except for the ones at the boundary. The unequal attraction of all the molecules that are at the surface forms a 'skin'. When you look at this picture you see that the paper clip forms a depression in the surface of the water. If the pressure exerted by the paper clip is low enough, then the surface tension can support it, therefore making it float.