Yes, it's plant flesh that contains seeds. But hear me out.
The other day my 8-year-old daughter came home from second grade, flush with the joy of new knowledge: The tomato is actually a fruit!
How did she know this? Simple: It has seeds. Foods that have seeds are fruit. Ipso facto, tomato = fruit.
This kind of definition-by-reduction is, of course, the source of many great faux-fights on the internet (A hot dog is a sandwich! Cereal is soup! Cheesecake is pie! A corndog is... a popsicle? Seals are dog mermaids.)
But is a tomato a fruit? And for that matter, what about cucumbers, eggplants, olives, and pumpkins? Oh my God, is everything but celery basically just fruit? No, of course not. If you order a bowl of fruit salad at a restaurant and any of those appeared in the dish, you'd be (rightfully) upset.
It turns out this argument is much older than the internet. Back in 1887, the U.S. decided to impose tariffs on imported vegetables—but not on imported fruits. So a tomato importer named John Nix sued, arguing that he was importing fruit. This argument made its way to the Supreme Court, which ultimately decided in 1893 that a tomato was, yes, a vegetable—and Nix had to pay his taxes.
The basis of their decision? It hinges on two different sets of people both using the word "fruit" in similar, but slightly different ways.
To a botanist in the field, a fruit is the part of a plant that deals with reproduction—the seeds and flesh around those seeds. (Let's not get into the confusion of berries or drupes for goodness sake.) But bean pods, for instance, are botanically a fruit.
And botanically speaking, a "vegetable" of course means... well, nothing at all. It doesn't describe any one part of a plant. It's a word that pretty much only has meaning in the kitchen.
In the kitchen, of course, a vegetable is any (generally savory) edible part of a plant—sometimes the leaves, sometimes the root, or the stalk. And a fruit is any (generally sweet) dessert-like edible part of a plant: Apples, strawberries, peaches, and watermelon all fit the bill.
Yes, most of those are also botanically fruit, but not all of them. Figs, for instance, are culinary fruits, but botanically speaking, they're more like flowers. Thus, the Supreme Court said: "As an article of food on our tables, whether baked or boiled, or forming the basis of soup, [tomatoes] are used as a vegetable...." And yet, despite that ruling, the argument continues.
So why does confusion persist? Why is this, more than 100 years later, still an ongoing argument?
I think there are a couple reasons. One is that we humans are just so delighted by the idea of secret knowledge—of being "in" on an idea that seems counterintuitive, that we reflexively grab onto it, and then happily bring it out to share with our parents (or to post online) in order to show a bit of intellectual astuteness.
We like the idea that while everyone else goes about their daily lives blithely making BLTs with what they assume is a vegetable, those of us in the know realize we're all actually putting fruit all over our sandwiches. (Ha ha! Weird!)
In this way, I suppose, I'm doing the same thing. (Ha ha, you tomato-fruit thinkers! Let me drop some knowledge about botany and the Supreme Court! It's counter-counterintuitive!) But there's another reason we like to argue over stuff like this, especially online: It's an easy, low-risk fight.
The internet is a difficult place for making the kind of connections that lead to mind-changing insights. There's a lot of distraction, a lot of noise, and it's hard to engage with other people in a deep or earnest way.
But it's an easy place to have arguments. People "battle" constantly across Twitter threads, Reddit message boards, and Facebook comments over an endless variety of subjects—to say nothing of the vast number of opinion pieces on websites large and small. It can be enjoyable to see someone you agree with saying something witty or cutting—but when the stakes are high, seeing arguments play out can also be scary, or upsetting.
However, with nonsense arguments (about food definitions, for instance), we all get to take a side and then play out our arguments, knowing there's nothing more at risk than what you're going to call a food that's decidedly delicious and nutritious. It's all of the fun, with none of the fear. And in that case, if you want to call a tomato a fruit, go ahead: Come at me. I can take it. It'll be fun.