1. You created it/him/her (unless you're baby-sitting. In that case, DO NOT CLAIM THIS BABY AS YOUR OWN, ESPECIALLY IF YOU'RE A TEENAGER LIKE I AM BECAUSE THAT GIVES PEOPLE A HORRIBLE IMPRESSION. And it's pretty darn weird. And mean. And okay. Just don't plagiarize.)
2. It revels in destruction.
3. It laughs, you laugh. Unless it makes bad jokes. Babies don't usually make jokes, though. Well, I don't know, maybe they do in their own language. If you have a baby making jokes that you can understand, call a scientist. And maybe Good Morning America.
5. It doesn't always understand the meaning of danger. "No, don't bite that electrical cord." "No, don't date a hungry vampire." "A staircase and slide are two entirely different things!"
6. It can't be all by itself for an extended period of time. Give it a sidekick or a pet or an enemy.
7. It can make you gain weight. Even if you aren't pregnant, you may often find yourself reverting to chocolate as a form of stress reliever to all things MC/evil child.
8. You want some quiet time? Too bad.
9. It may not always obey your every command. It may never obey your commands. It may even do the exact opposite of what you command.
10. It likes to break things, be it your mother's favorite vase or another character's heart.