There was a bit of confusion in the office today. What is a tank? As it turns out, many of the things that you think are tanks are, in fact, not.

What you need to know is that what makes a tank a tank is not what the thing looks like, but how it’s built and what it’s built to do.

To explain what I mean, here are some things that are not tanks:

That’s not a tank, it’s an M113 armored personnel carrier.

That’s not a tank, that’s an M93 Fox Chemical Biological Radiological and Nuclear reconnaissance vehicle.

Though this really looks like a tank, it is not. That’s an M1128 Stryker Mobile Gun System.

And this is not a tank, despite looking really incredibly very much like one. It’s a Bradley Fighting Vehicle.

On the other hand, these are some tanks:

That’s a tank. It’s a Danish Leopard 2.

That’s a tank. It’s an Israeli Merkava.

That’s a tank. It’s a Swedish S-tank, also known as the Stridsvagn 103.

At this point you probably think I’m just randomly picking vehicles and declaring some “tanks” and the others, arbitrarily, “not tanks.” But there genuinely is a methodology here, and it’s not just pulled out of a hat.

Somewhat.

I will say, upfront, that what is or is not a tank is often determined by various militaries . Again, that’s often backed up by not what the thing looks like, but how it’s built and what it does.

Tanks, first and foremost, are:

Extremely heavily armored, often weighing upwards of 60 tons as a result. Equipped with a big gun, usually in at least 120mm caliber in modern tanks. Tracked, rather than propelled by wheels and tires.

And finally, their primary purpose is to fight and survive.

Take that Danish Leopard 2 pictured above, for instance. It weighs 68.7 tons, largely thanks to massive amounts of steel plate and composite ceramic armor, designed to (maybe, depending on who you ask) defeat a direct hit from another tank. It carries a crew of four—a gunner, a driver, a commander, and a loader—and no more. Its 120mm smoothbore cannon can fire a mix of rounds, from high explosive to armor-piercing sabot rounds.

It’s got tank treads.

Most tanks have turrets, too, although there are exceptions like the Stridsvagn, which had its gun stuck in place but used hydraulic suspension to move it up or down. But despite the lack of a traditional turret, it still meets the “tank” definition because of its heavy armor, big gun, tank treads, and frankly because the Swedish government said it was.

The problem with defining a tank, however, is that it’s easy to say why something is a tank, but it’s harder to say why it’s not a tank, other than it doesn’t meet the specifications. As ever, it’s difficult to prove a negative.

So we’ll use the examples from above. Take that M113, for example:

The M113 isn’t really meant to take on the enemy in its own right, as it’s what’s known as an Armored Personnel Carrier (or APC). Oh sure, it’s got that M2 Browning .50-caliber machine gun up top, and while that’s frankly useful for killing people, it wouldn’t stop a tank one bit. (There are variants that have different weapons on top, like grenade launchers.) And while its metal plating might look impressive to the casual observer, it’s only meant to stop small arms fire such as an AK-47, but rounds from something like a heavy machine gun would have a good chance of slicing right through.

If you’re wondering why the M113 exists at all then, it’s because the main job it was originally designed for is getting troops to the battle from a base, and then getting them back out again. The M113 is designed to carry personnel, while being armored. That’s why it’s an APC. Instead of carrying crew of four, it carries only two and makes up the difference with room for 11 passengers in the back. The reason why it’s so lightly armored (in comparison to a big, heavy tank) is because it needs to be easily transportable on something like a C-130 cargo plane.

Once more, let’s apply those rough rules of tankdom to the M113 APC:

Heavily armored: Not really. It is lightly armored.

Big gun: Halfway there. Its .50 cal is nowhere near as big as a true tank’s.

Tracks: Yes. Its tracks are small, but they’re there.

Designed to fight and survive: Not really. It’s designed to get troops in and out of the middle of a battlefield. So can it fight against a guy with an AK-47? Sure. But anything bigger and it’s going to have trouble.

That set of rules goes mostly the same way for the rest of the vehicles on our list. They’re either APCs or based on APCs. The M93 Fox, for instance, is meant to scope out nuclear battlefields, and it’s based on the TPz Fuchs armored personnel carrier of the German Army. The M1128, despite having the same 105 mm cannon originally fixed to the American M1 Abrams tank, is based on the . And while it is more heavily armored than an M113, it comes nowhere close to the amount of protection a veritable tank offers.

That Bradley? Well, that’s an unusual case. While it looks like a tank, with the treads and the turret and everything, it only weighs about half as much as a tank. Instead of a full-size tank cannon, it uses a 25 mm M242 Bushmaster autocannon. And while that probably won’t stop a tank, it’s still got enough punch to mess an average person’s day up (:this is a video of an M2 Bradley firing at enemy snipers in Iraq. People almost certainly die in this clip. So just a heads up):

So if a Bradley looks like a tank, has a turret like a tank, and has treads like a tank, but isn’t a tank, why the hell did the U.S. military see it fit to make it so much like one?

Well. It turns out the Bradley is sort of an in-between mish-mash of sorts. The M2 Bradley was originally supposed to be a replacement for the M113. It was meant to be lightly armored, lightly armed, infantry transport. And then the military procurement process happened, and it turned into something else entirely, as beautifully explained in this clip from the HBO movie The Pentagon Wars:

But it’s still not a tank.

Hopefully, though, now you know what is one.