Why can a spider needle float on the surface of water even though the metal of the needle is

about 18 times denser than the water?

Because of the surface tension of the water

Explanation:

A metallic needle is denser than water. Therefore, when it is placed in water it must drop down to its base. But this usually does not happen. This is so because the water molecules at the surface of the water are strongly attracted towards each other. This molecular force creates a surface tension on the surface of water that prevents metallic needle from drowning.