Penicillin was one of the first antibiotics and is probably the most famous. Penicillin comes from the Pencillum fungi (mold.) In the case of penicillin, the anti-bacterial stuff oozed out from the mold. So I guess you could say it was the mold juice. The way penicillin was discovered was actually by accident. A scientist had some bacteria growing on a plate and noticed that it had been contaminated by mold. He noticed that around the mold the bacteria didn't grow. He collected the mold juice and did a few more tests and ... penicillin was born! However there are more types of antibiotics besides penicillin. Other antibiotics can be made in other ways as well. A popular method of finding antibiotics is targeting a specific part of the bacteria that human cells don't have and then looking through thousands of chemicals (which may come from a mold/plant/etc or synthesized in a laboratory) to see if they can find one that kills bacteria but doesn't kill human cells.