At school, we're taught that there are three basic states of matter: solid, liquid, and gas. But out in the real world, it's often not as easy as you might think to work out which materials are in what state. Take glass, for example. It looks and feels solid, but on the atomic level, it kind of looks more like a liquid. So how do we classify it? Derek explains in the latest episode of Veritasium.

You might have heard people talking about glass being a liquid before, as the idea is often used to explain why ancient stained glass windows are thicker at the bottom - over time, some people believe, the glass has very slowly been flowing downwards.

It sounds like it could make sense, but as Derek explains, this just isn't true. In fact, we can still use telescopes from hundreds of years ago to accurately look at the stars, because glass holds its shape so incredibly well.

So what is it about glass that makes people think it's a liquid? It all comes down to its atoms, which aren't arranged in a nice crystalline structure like they are in most solids. Instead, they're all jumbled up, which you'd usually see in the structure of a liquid.

But glass is a special type of solid, called an amorphous solid, which means that it cools down so quickly from a liquid state that its molecules don't have time to arrange themselves properly.

As Derek explains, it's not actually the arrangement of the atoms that determines whether a material is a liquid or a solid, it's how strongly they're chemically bonded together. So if glass was a liquid, its atoms would be able to slide past each other at room temperature, but in glass that doesn't happen.

It's not just glass that's confusing, there are liquids out there that behave more like a solid, like pitch. Pitch is a super-viscous derivative of tar, and it's the subject of the world's longest running lab experiment at the University of Queensland in Australia - The Pitch Drop Experiment.

Back in 1927, scientists placed the pitch in a funnel over a beaker, and have been monitoring it ever since to see how long it would take to flow downwards. In the 88 years since, it's only dropped nine times, and no one's ever been in the room to see it happen.

In fact, you can monitor the experiment yourself via live stream... but with the last drop happening in 2014, you might be waiting a while to see some action.

But what about Earth's mantle? That feels pretty solid, but when it comes to plate tectonics and earthquakes, it can also move incredibly fluidly.

Again, the answer's not so simple. Watch the video above to find out, and be prepared to have everything you thought you knew about states of matter blown out of the water. Thank you, science.