Why does hydrogen peroxide foam when you put it on a cut?

Hydrogen peroxide does not foam in the bottle or on your skin because there is no catalase to help the reaction to occur.

Hydrogen peroxide (H₂O₂) is something you can buy at the drug store. What you are buying is a 3-percent solution, meaning the bottle contains 97-percent water and 3-percent hydrogen peroxide. Most people use it as an antiseptic. It turns out that it is not very good as an antiseptic, but it is not bad for washing cuts and scrapes and the foaming looks cool.
The reason why it foams is because blood and cells contain an enzyme called catalase. Since a cut or scrape contains both blood and damaged cells, there is lots of catalase floating around. Read More.