[siddharth]

I read that to place a pin on a water surface such that the surface tension of the water film keeps it afloat , it helps if you first grease it.( say by rubbing it against your forehead) How does that extra layer of oil help? Does it modify the surface tension of the water somehow. Any help would be greatly appreciated.

[fledarmus]

Water displays surface tension because the molecules of the water would rather interact with other molecules of water than with the molecules of whatever is outside the water - in this case, the metal of the pin. However, if your pin has been washed and has some soap remaining on it, or possibly salts from contact with perspiration or corrosion, then this isn't necessarily the case. Adding a layer of oil to the outside of the pin ensures that the surface of the pin will not have any favorable interactions with the water.

[siddharth]

Thanks. That cleared it up.