We're checking out at the grocery store when the clerk asks the inevitable: "Hey are you two twins?" Yup, Caitlin and Ellie, 10, are twins all right (fraternal, in fact, yet nearly mirror images of each other). If I had a buck for every time we've heard this question, I'd be on the Forbes list alongside Bill Gates and Warren Buffet. Yet the girls never tire of it, nor of the "Wait, which one are you?" they hear over and over from classmates, teachers, friends, cousins, and occasionally -- gasp -- their own mother.

It's as predictable to them as a long, cold winter in Maine, and it's part of the price of the celebrity status that twins enjoy. And what kid wouldn't put up with the occasional annoying question in exchange for a little Tom Foolery?

One morning when the girls were 6, Ellie, who is rarely up first, came down to breakfast in a nightgown that both girls had, hair rumpled. I called her Caitlin for 20 minutes before she told me she was, in actuality, Ellie. She still needles me about the memory. On April Fool's Day in third grade, the girls swapped not just clothes and backpacks, but classrooms, too. They had their teachers going through math and half of spelling before the jig was up. Then there was the grade-school boy who had a crush on Ellie. He shyly squeaked, "Hi," to her all year in the halls. Or so he thought. Half the time he was talking to Caitlin, who never let him in on the fact that she was her sister's twin.

But it's not all laughs to have a sister that looks just like you. At our house, it's a constant effort to make sure we and others treat the girls as the individuals they are, and not lump them conveniently into a tidy twosome. Since kindergarten, Matt and I chose separate classrooms to foster independence, encourage the girls to make their own friends, and grow and learn out of each other's shadow. Sounds good on paper, but hard to pull off. It's almost irresistible for us and teachers not to compare their progress, although the intent is never to pit them against each other, but to make sure that both girls are moving steadily forward. There are social challenges, too. In first grade, one of the girls was invited to three birthday parties in a row, leaving the other feeling left out and hurt.

Now that they're in fifth grade, even their peers compare them more intensely. The other day, they replayed a lunchroom conversation that left me torn. "They said I was prettier and smarter," reported Ellie. "But they said I'm nicer and funnier," Caitlin quickly added.

What to do? We encourage Caitlin and Ellie to have both shared and separate friends. We highlight and celebrate what's unique about each of them. Ellie has a memory like Velcro, a knack for playing clarinet, and a sensitive nature; Caitlin tends to think out-of-the box, loves action of any kind, and is particularly attached to her toy monkey, Nanas.

But we don't ignore what they share -- like a passion for skiing at top speed through trees, a tendency towards all things silly, and a general joi de vive. We make time for one-on-ones, even if it's as simple as a game of checkers, a hot chocolate date, or a run for groceries. As Caitlin and Ellie become tweens and then teens, I suspect this will become more important than ever. Then again, there's always purple hair dye, wild tattoos, and body piercing to show the world who you really are.