Just a couple drops. Hydrogen peroxide is an unstable chemical compound - essentially water with an extra oxygen. When added to water, the extra oxygen molecule literally explodes off, leaving water with a little extra oxygen in it. Good for growing plant roots in water.
Too much though, its caustic to living material. Have you ever poured hydrogen peroxide on a cut? Fizzle, fizzle. That little exposition is busy destroying the healthy tissue from around your cut.
