• Matter is the word for everything around you. It's anything that has weight and takes up space—from your phone and the air you breathe to the stars in the sky. In our analogy, matter is all the LEGOs you could have, of every shape and color.

• An atom is the tiniest, most basic building block of everything. You can't break it down any further by chemical means. Think of an atom as a single, individual LEGO brick.

• An element is a pure material made up of only one type of atom. Imagine you have a big pile of only red 2x4 LEGO bricks. That pile is like an element, such as pure oxygen or pure gold.

• A molecule is what you get when two or more atoms stick together. It doesn't matter if the atoms are the same or different. A molecule of oxygen gas is two oxygen atoms bonded together, a`nd a molecule of water is two hydrogen atoms and one oxygen atom. So, sticking any two LEGO bricks together makes a LEGO molecule.

• A compound is a special type of molecule. It’s what you get when you bond atoms from at least two different elements. Water is a compound because it has both hydrogen and oxygen atoms. So, if you connect a red LEGO brick to a blue one, you've made a LEGO compound. An oxygen molecule, made of two oxygen atoms, is a molecule but not a compound.

• A substance is just a particular type of matter that's pure and has the same stuff all the way through. Both elements and compounds are considered pure substances. So, a pile of pure gold (an element) is a substance, and a glass of pure water (a compound) is also a substance.

Air has weight, and it's always pushing down on us and everything around us. This is called air pressure. For water to boil, it needs to get hot enough for the water vapor bubbles to push back against that air pressure and escape.

At sea level, water boils at 100°C. But if you go up a mountain, there's less air above you, so the air pressure is lower. With less pressure pushing down, it's easier for the water bubbles to escape, so the water boils at a lower temperature.

It's important to know this because it affects cooking. If you're camping on a mountain and boil an egg, the water might be boiling, but it's not actually as hot as it would be at sea level. This means your food will take much longer to cook.

A pressure cooker does the exact opposite. It's a sealed pot that traps steam inside, which dramatically increases the pressure. Under all that high pressure, the water has to get much hotter than 100°C before it can boil. Because the water is super-hot, it cooks food much faster than a regular pot.

It’s not a good idea to use mass to identify a substance because mass just tells you how much "stuff" is there, not what kind of stuff it is. For example, a pound of feathers has the same mass as a pound of rocks, but they are obviously very different materials. Mass changes depending on how much of the substance you have.

A much better property for identifying a substance is density.

Density is a measure of how much mass is packed into a certain amount of space (its volume). Think of it as how "crammed together" the stuff inside an object is. Every pure substance has its own unique density that doesn't change, no matter if you have a tiny piece or a giant chunk. For example, a small gold flake and a large gold bar have the exact same density.

Because each substance has a characteristic density, you can figure out what an unknown material is by measuring its mass and volume, calculating its density, and then matching that number to a chart of known substances. It's like a fingerprint for matter.