When you drop a watermelon off a 45-metre (148-foot) tower, you expect it to fall fast and leave a big mess at the bottom. And that's exactly what happens, most of the time.

But as the latest episode of Veritasium shows, if you spray that watermelon with a weird, black polymer coating first, it doesn't actually break when it hits the bottom, it… bounces.

You can watch it for yourself below:

Ummm, what?

Here's that bounce again in gif form, if (like us) you can't get enough of it:


Not even an axe can break the coating apart. And when the team from How Ridiculous - who provided the footage of the watermelon experiment - finally crack it open with a buzz saw, they find that, apart from being incredibly slushy in the middle, the casing of the watermelon is still remarkably intact.

So, what the heck is going on here? Because anything that can stop a watermelon from breaking after a 100 km/h (62 mph) fall sounds kinda like sorcery.

In reality, it's all thanks to a type of spray-on truck bedliner coating called Line-X, and it's the same stuff that's used to line the walls of the Pentagon, and make bullet-proof vests.

As Derek from Veritasium explains, Line-X works so well because it's pretty much a big mess. Its internal structure is a mess, the ingredients are a mess, and it goes on as a mess. 

But the end result is pretty spectacular.

It's made up of two ingredients, handily nicknamed A and B.

A is mostly made up of a molecule called Diphenylmethane-4,4′-diisocyanate, or MDI for short, which is very reactive at its ends. 

B is the plasticising part, which has a molecular name so long we're not going to type it out, but you can see it below:

Screen Shot 2016-12-07 at 5.41.01 pmVeritasium

In reality, the molecule is a lot longer than it looks in that image, and when A and B mix they bond together within seconds, forming an even longer molecule known as a polyurea.

These long, tangled molecules give Line-X its strength, but also its flexibility, because they can be stretched out and snap back into place.

As you can see in the video above, when the reaction first happens, the coating is incredibly hot because the binding of these molecules releases so much energy as heat.

At first, you can still pull the coating apart. But after a few minutes, it gets really solid, really fast - Derek can't even rip a piece of paper coated in the stuff.

So how does all of that work to protect a watermelon being dropped off a really tall tower?

We'll let Derek explain the physics of that one to you in the video above. But it's fair to see that Line-X is our new favourite material of 2016, and we kinda want to spray it on everything now. Indestructible iPhone, anyone?