When aggravated by a predator, they, you know, puff up.

Some puffers, like the porcupine fish, become a bona fide spike ball, moving through the water seemingly out of control.

But if you peer inside a puffer, you'll learn that puffing up isn't the only trait that makes these fish one of the most threatening creatures in the sea.

Contrary to what it looks like, puffer fish are not like balloons.

Because what's normally inside them isn't air; it's water.

What they do is they actually take water into their mouths in a big mouthful of water, and then they pump it down into their stomach.

That's Elizabeth Brainerd, a biologist and puffer fish expert at Brown University.

And they do that anywhere 10 or 15 times; pump, pump, pump, pump, pump, pump, until they inflate completely,

and then they hold it and they'll just be a big, spiny ball.

And as you might expect, this requires some pretty sophisticated biology, starting with the stomach.

It's made of dozens of tiny folds, kind of like an accordion.

These folds are important because when the stomach fills up with water, it can expand without rupturing.

And puffer fish expand a lot; up to three times their size.

That's like if an average human man could inflate his waist to a circumference of 3 meters.

But there is a drawback to these amazing skills.

Brainerd suspects that puffer fish stomachs have actually lost the ability to digest food, which means their intestines have to do all the work.

You know, given the apparent importance of this defense mechanism, they've given up the advantages of having a stomach where some digestion can start.

But the stomach?

It's just one of many bizarre features inside a puffer.

For example, they have specialized muscles that you won't find in most other fish.

Some in their mouth, which pump all that water into their stomach;

some in their esophagus, to seal off their stomach like a drain plug once it's full;

and some in the base of their bellies, which contract to squeeze out water when they're ready to deflate.

But what you won't find inside is even more bizarre.

There are a couple characters that are really helpful in their ability to puff up,

and one of those is that they don't have any ribs, and another one is they don't have any pelvis.

In other words, puffers are essentially missing bones.

And that's a good thing, because otherwise they'd get in the way of inflation.

In fact, according to Brainerd, if it weren't for these missing bones, puffer fish would probably have never evolved this way in the first place.

And that would be a shame, since puffing up really is a good defense.

Consider one old study in which researchers watched birds go fishing.

The birds caught 11 puffer fish, but they dropped nearly half of them because the fish started to inflate.

But what's more surprising is that the birds left with empty beaks might have been the lucky ones.

Because puffers have another, more potent defense up their sleeves.

Their bodies are laced with a neurotoxin called tetrodotoxin.

It's up to 1,200 times more poisonous than cyanide.

So poisonous that one puffer fish can kill 30 adult humans.

So poisonous that puffers are reportedly the second most poisonous vertebrate in the world, which is why it's also surprising that us, humans, we actually eat them. That's right.

In Japan, puffer fish is actually a delicacy called fugu, which only trained chefs can prepare.

And considering that these fish are basically spike balls filled with poison and we're still serving them in restaurants, they must be seriously delicious.