Why the soap can become foam?

To put it simple, foam is actually water-detergent mixture.

Detergents have a polar head group and a hydrophobic tail group.

When put in water they form micelles. (You can simply take that as a group of soap molecules coming together.)

These micelles decrease the surface tension of water and aggregate to form foams.

2009-05-17 03:55:31 補充：
"water-detergent mixture" => "water-detergent mixture with air inside"

because soap is a so-called wetting agent.
It can decrease the surface tension of water,
allowing foam / bubble to be formed.