Percival and the Brain continues, as our best friends navigate the murky, tumultuous waters of puberty.

Will their friendship survive? Tune in next fortnight and see!

There are some things about being a teenager -- well, a lot of things, actually -- and all of them are strange, and I think, as I have mentioned before, that most adults must sort of block those years out of their memory, or perhaps cover them over with a peach-colored haze of nostalgia. Were you ever, as a 15- or 16-year-old, told "these are the best years of your life"? Did you think, "thanks, Grandma, but that's one of the cruelest things anyone has ever said to me, and if I actually believed you, which I don't, I would have no choice but to kill myself right now."

Different people lurch through puberty on completely disparate timelines. Sometimes friends get out of synch, and sometimes they synch up again later. Sometimes they don't, and sometimes this leaves behind weird, bitter, painful residues. Falling out of love with your best friend can be as painful as losing the other kind of love. But when your relationship was based on sharing Barbies and horses, how does that work out when you enter adolescence and neither of you cares so much about Barbies anymore, but one of you remains highly religious while the other becomes a skeptic? Or perhaps (a little later in teenagehood) your relationship is based on a shared obsession over Lord of the Rings and the Beatles, but one of you strikes out from there to discover Stephen King and punk rock, while the other goes in a Terry Brooks, Mannheim Steamroller direction?

This kind of drifting apart doesn't stop in adolescence -- adult friends break up in much the same way -- but when you're a teenager, all of this stuff is happening to you for the first time and you don't always know how to handle it.

The very definition of teenager -- a social distinction having no basis in biology -- is someone who is neither a child nor an adult. In other words, someone who has no place in the world.

We created teenagers because our society is judged too complex for physical maturity (and a single well-defined ritual, for example, killing a bear) to be the only barrier to adulthood. Instead, we have a series of arbitrary gates that people stagger through: at 10 or 11 your school sends you to a sex education and shows you embarrassing films unless your parents have a religious objection, at 16 you can drive, at 17 you can see an R-rated movie, at 18 you are technically an "adult" but you still can't drink until you're 21 -- so when are you actually an adult? When you're 21 and you can go into bars? When you're 35 and you can run for president? When you start acting like an adult? When you get married, buy a house, have a kid?

Except, a lot of us could have had a kid -- in the technical, biological sense -- as young as 12.

And I'm pretty sure a 12-year-old isn't an adult. Is she?