點解一張紙可以被承托喺水面? 超急.!

You are right. This is because of surface tension, not floatation.
Due to strong cohesive force between water molecules, a water molecules is attracted by all other molecules areound it. But for those molecules on the surface, there are only molecules on the sides and below. There is no water molecule above the surface. Because of the absence of choesive force from above, this gives the result that molecules on water surface are pulled downward by cohesive force of other molecules. This makes the water surface under a form of "tension" and behaves just like a stretched balloon skin.
A paper placed on the water surface is therefore supported by the "tension" developed by the "stretched skin" of the water surface. Should the "skin" be broken somehow, the "tension" of the skin would disappear and the paper is no longer supported by the water surface.

表面張力現象 Phenomena of surface tensionhttp://www.youtube.com/watch?v=Ays36S8hd2U

http://www.youtube.com/watch?v=vmpySCTM3LA

http://www.youtube.com/watch?v=Xmir2h72-jg