You've most probably cut yourself, been grazed or fallen and hurt your knee or had a sharp object driven into your foot at some point...
When you did this, I'll bet someone told you to put some oxygenated water on it to cure it, and you didn’t like it because it STUNG!!!!!
Well, the fact that it stung meant that your body was reacting.
The story of small wounds and how to cure them is extremely interesting.