You know the best thing about being an only child? There’s no math involved. No fractions required to divvy up the last piece of cake. No pie chart needed to see who got the most TV time.

Sibling-free, I got it all. All the love. All the attention. I got praise for the academic subjects I mastered, like French, and even those I didn’t, like trig. When there’s no competition, you get kudos for succeeding at arithmetic as simple as this: Love divided by one is one.

It wasn’t until I was an adult — and pregnant — that it first occurred to me that love might have a numerator and denominator. My husband and I worried how our beloved dog would cope with having a cooing, pink love-hog in the house. Isn’t it a crime to lavish affection on something and then ask it to share that affection with someone new? I asked our vet.

“Love grows,” he said.

“What does that mean?” I asked with a seriousness that should be reserved for conversations about heartworm and distemper.

“The heart expands,” he purred cryptically. He was one of those hippie earth-father vets with tons of his own kids and a fluffy, wisdom-indicating beard. “Love multiplies.”

Damn it! There would be math.

The dog did get less attention from us; of course she did. But she drew new regard from the baby, who became a toddler, who became a boy, who loved her more than anyone else ever had. Or ever will.

When I became pregnant again, I fretted anew. Wildly in love with my firstborn, I couldn’t fathom how I’d find room in my heart for another child. I literally wept with worry. I asked a friend with two daughters how it was supposed to, you know, work.

“You love them equally,” she said, “but differently.”

It sounded like nonsense to me. Utter gibberish. About as practically useful as the Pythagorean Theorem.

But I’ve discovered since then that she was right. I don’t know if my kids will ever ask me aloud which one I love most. (Do siblings actually do this? The horror!) There was a time when I would have told them that whoever asks that question is the one I love least.

I have a different answer now. One that’s better because it’s true. There’s not a measuring instrument known to science that could detect even the slightest imbalance in the amount of heart-swelling adoration I feel for each of my boys. But I do love them for different reasons, in different ways. I do love them differently.

There are distinct kinds of love. Allegiance. Admiration. Appreciation. We treasure our mothers differently than we treasure our spouses; why wouldn’t we cherish our children in unique ways, too?

I love one of my sons for being such a great role model, and the other for being such a quick study. I love my first child for teaching me to be a mother, and my second for showing me that, well, motherhood isn’t something you ever really master.

I love my oldest for demonstrating the cool stuff that happens when you mix my DNA and my husband’s in the biological blender; I love the youngest for proving that said blender also has snazzy settings for chop, whip, and liquefy.

I adore my boys for educating me in the basic principle of motherly math — that while our devotion may be immeasurable, it’s not incalculable. It turns out that parenting a pair isn’t love divided by two. It’s love squared. It’s an equation so simple that even I can understand it.

But dear god don’t give me three.