Ever cut yourself, have someone put a dab of hydrogen peroxide on the cut, watch the entire area fizz like something that would turn you into the Joker, and wonder, "How is this good for me?" A simple chemistry demonstration that you can do at home explains it all. Take a look at foam snakes.
Foam snakes, or elephant toothpaste, is a pretty simple chemistry demonstration. It takes nothing more than a container, a little dish soap, some yeast, a bottle of hydrogen peroxide, and perhaps just a touch of food coloring, if you want to get decorative. What it produces is a massive and harmless jet of foam that will shoot out of the beaker at you like it has gained sentience and knows you killed its mother. Unfortunately for it, it can't do you any real harm, other than getting soap in your eyes, which is why it is demonstrated in classrooms the world over.
The chemical formula for hydrogen peroxide is H2O2. That's water with one extra oxygen atom attached to it. Putting in dish soap and food coloring liquid doesn't do anything to the hydrogen peroxide. Putting in yeast does. It starts off a process that goes at a rate of about 200,000 reactions per second. Inside the yeast is an enzyme called catalase. The enzyme, along with an extra oxygen atom, rips an oxygen atom off the hydrogen peroxide molecule, leaving oxygen gas and H2O, or water. The extra oxygen atom is that has just been freed up helps in another reaction and the whole thing spreads. What's left is a lot of oxygen gas and water. The water has already mingled with the soap that was mixed in at the beginning of the experiment, and creates foam. A lot of foam. The foam bubbles up and shoots out of the container, amazing and delighting students, or at least shaking them out of their stupor for a while.
Anyone who has dabbed hydrogen peroxide on a cut has seen a tamer version of this bubbling. The mixture fizzes in an ominous way before subsiding. This is because catalase, the same enzyme that was in the yeast, is also in human blood. (It's in most blood, in fact, but few demonstrations using blood to make this kind of foam have been found online. Volunteers?) The bubbling that you see is the catalase in the blood breaking down the hydrogen peroxide into water and air bubbles.
We have this stuff in our bodies because natural processes create hydrogen peroxide in the body, and in the body is exactly where it should not be. When hydrogen peroxide breaks down, it oxidizes, adds oxygen to, the molecules around it. Since modifying molecules with almost any extra atom makes them change completely and alters their function, this can kill cells. Killing cells is why hydrogen peroxide gets dumped on superficial cuts in the first place. It oxidizes and kills bacteria in the wound. This oxidization makes it bad for treating internal wounds, and is why it shouldn't be used too frequently. It can cause damage to internal cells, slightly increasing the chance of cancer over time. It can also destroy healing white blood cells. However, a little dab every now and then on a superficial cut doesn't do any harm. And it's nice to have on hand if you ever want to do any chemistry demonstrations.
Note: You can also do this experiment with potassium iodide solution, but it's harder to get than yeast.