Humans like to reproduce. It's a well-known biological fact. In the past fifty years, the world's population has grown by roughly four billion people. That's a lot of new kids out there in the world who need a loving parent to wash them, dress them, berate them, and maybe even love them a little bit if they're lucky.

But for some reason, even though humans like reproducing little humans, they don't always like taking care of them. Gosh, if only they could just find someone else to take care of the young'uns, even for a couple hours a day.

That's where nannies come in. As a nanny or an au pair, you get paid a small salary, and room and board, to pick up these parents' slack. Sounds like a second childhood, right?

Wrong. In fact, you should think of these homes as nicely-furnished offices where you happen to pull all-nighters every night for a couple of years on end. After a couple of days on the job, chasing the little tater tots around the playground, you're going to be praying for just ten minutes in a cubicle where you can shut yourself in, all alone with your thoughts, where the kids can't find you…where no one can find you.

Don't even think about getting comfortable. First, this isn't (Insert Your Name Here)'s Day Off. Just because the parents are out of the house doesn't mean you get the run of the place. We hate to break it to you, but as a nanny you'll be an adult getting paid to do a really hard, really exhausting job for another adult. You're probably thinking: "How hard and exhausting can it really be? I used to babysit!"

Um, yeah. Comparing babysitting to nannying is the same thing as comparing apples to…door hinges. They're completely different things. First of all, as a nanny you'll have longer hours. If you were the sort of babysitter who showed up after the kids had already gone to sleep and spent your time "babysitting" by scavenging through the parents' kitchen cupboards, you may as well wave a white flag and surrender now.

Because when you're a nanny, the kids you're taking care of are going to be fully awake in all their whining, crying, temper-tantrum-throwing glory.

Second, as a nanny, your job might be as boring and monotonous as an assembly line worker in a factory. Except this factory is dedicated to producing well-rounded kids, not the heads of toothbrushes.

Your days are going to be spent taking the kids to carpool, picking them up from school, going to the park, fixing their meals, fixing your own meals, counting slowly and patiently to a million before you lose your cool, tricking the kids into brushing their teeth, and then tricking them into falling asleep. With all due respect to the Trix rabbit, tricks aren't for kids. They're for nannies who need to take care of bratty, snot-nosed kids who refuse to go to sleep.

Speaking of bratty, snot-nosed kids…if you're looking for a job that will stretch your imagination and test your mental powers, taking of care of kiddos ain't it. Sure, being a kid was fun while it lasted. But last time we checked, we weren't living in Neverland, and you weren't Peter Pan. Which is to say, you've grown up. And because you've grown up, it's going to be much, much harder than you think to be an adult hanging out with little kids all the time.

Don't expect a ton of intellectual fulfillment as a nanny. Unless for some reason, the kid you're taking care of likes to solve Rubik’s cubes in his spare time, and to save his hometown with all his zany experiments gone awry, à la Jimmy Neutron. Ah, but Jimmy Neutron never had a nanny, did he? Nope. So move along.

In fact, you'll be doing a lot of "moving along" as a nanny. Though the nanny market is a pretty stable one, the average nanny job only lasts for a couple of years. If you've ever seen Mary Poppins, which we all know is basically a behind-the-scenes exposé of what it's really like to be a nanny, then you know this already. Even the practically perfect Mary Poppins only stayed until the wind changed directions. So if you're looking for a steady job as a nanny, fingers crossed you live in a city where the wind never changes direction.

Ability to make friends with cartoon forest animals optional, but preferred. Now that we mention it, make that always preferred, whether you're a nanny or not.

Clearly, being a nanny isn't a job for the faint of heart. So who is it a job for? Above all, being a nanny or an au pair is only a job for someone who loves kids. As a nanny, you'll probably spend the majority of your waking hours, five days a week, with little kids. If you're not careful, you might forget how to hold a conversation with adults. ("It’s been freezing lately!" "Speaking of which…Frozen was sooo good, right?")

So you should like kid-things.

You should also like taking care of people. The first aid section at the drugstore is where you really come alive. You approach treating ouchies and booboos with the same dedication some people have to solving the Sunday New York Times crossword puzzle. Because at the end of the day, being an au pair or a nanny is about keeping the kids safe, healthy, and happy.

Think of it this way: Dead kids make Mom and Dad unhappy. (They make most people unhappy, but Mom and Dad especially. And in this scenario, Mom and Dad are paying you.) Why do you think Mary Poppins took Jane and Michael's temperature after they got caught in the rain? Because she cares about the kids' wellbeing? As if.

You also need a lot of patience. Kids sometimes need to hear things again and again and again before they get it through their skulls. Right? Hello? Ahem, we said: Sometimes kids need to hear things again and again and again…. Oh, now you remember! Yeah, that can get annoying sometimes.

And we'd be remiss if we didn't use the "Real Poop" section to talk about how it might also help to have a high tolerance for snot, poop, and a whole host of smelly, sticky, all-around unsavory bodily functions.

Maybe nannying's not for you. What if you're considering becoming an au pair for a year or two? First, ditto to all of the stuff we just talked about. Annoying, disgusting kids, long hours, yadda-yadda. But as an au pair, you'll at least have the whole travel abroad thing going for you.

An "au pair" is a foreign national assigned to a family who can provide some sort of "cultural education" for their child. It's a legit thing. For instance, all au pairs entering the U.S. are screened by the U.S. Department of State. So what does that mean for you? It means that if you're good with kids and you speak English fluently, families in cool cities all over the world (Prague! Lisbon! Moscow!) want you.

Speaking of families…"au pair" in French means "as an equal." The idea is that the au pair joins the family, like the awkward Serbian exchange student your family hosted in ninth grade until she went missing and you never heard from her again. Anyway, you'll be that kid. You'll be included in family outings, meals, and vacations, which sounds cool.

Until you discover that the family you're stuck with is stranger than the Addams Family, more insular than the Brady Bunch, or zanier than the Simpsons. D'oh.

Whether you’'re thinking about becoming a nanny or an au pair, this isn't a job for the faint of heart. Anyone who's ever babysat a tired, cranky kid knows that they can be a real pain. We're talking food fights, temper tantrums, and meltdowns at that Despicable Me premiere showing you hoped would be fun for you both.

If becoming a nanny still seems like a great idea, we wish you all the luck in the world. But definitely don't expect life to be like a Julie Andrews movie. In fact, the first step in figuring out what it really means to be a nanny is realizing one simple truth: Julie Andrews lied to you. So go ahead and set aside those dreams of tea parties on the ceiling and pitch-perfect group sing-alongs on the Austrian mountainside for the next life, because they're not going to happen in this one.

• More