The problem is that the student didn’t take into account that there are some things that are too small for the human eye to see. Nearly everything in the world is covered with bacteria, viruses, and other microorganisms… most kids have done the “wipe a q-tip on a doorknob, then rub it on an agar plate and watch what grows” experiment in elementary school… It’s proof that there are microorganisms virtually everywhere, and when given the opportunity and the necessary resources (such as a good food supply on which to grow – like a piece of meat) – they can multiply and grow to a size that is visible to the human eye.

In short, the student didn’t just put a piece of meat in the jar, he put a piece of meat that naturally was coated in bacteria, fungus, and other stuff in the jar… it just took time for the microorganisms to grow to a size where they could be seen.

The meat became moldy

Meat is dead…it can’t reproduce. That’s jsut my guess.