Volume and capacity feel like two different worlds — one's about solid space, the other about liquid in a container. But they're bolted together by a single, lovely fact: a cube that's 1 cm on every side holds exactly 1 millilitre. That little cube is the seed the whole unit system grows from.
One Cube, One Millilitre
Picture a sugar-cube-sized block, 1 cm wide, 1 cm deep, 1 cm tall. Its volume is 1 cm³. If you could fill it with water, it would hold 1 mL. So 1 cm³ = 1 mL — not a coincidence, it's literally how the metric system was set up. That's the bridge between "how much space" and "how much it holds".
Stack Them into a Litre
Now build a bigger cube, 10 cm on every edge. How many little cubes fit? A flat layer is 10 × 10 = 100 cubes, and you stack 10 layers high, so 10 × 10 × 10 = 1000 cubes. Press "add a layer" in the toy and count them filling in. That's 1000 cm³, which is 1000 mL, which is 1 litre. A litre is just a 10 cm box of water.
So Where Do All the Units Come From?
Everything else is just bigger or smaller versions of that. A thousand millilitres make a litre; a thousand litres make a kilolitre (kL) — about a small backyard pool. On the solid side, a thousand litres of space is 1 m³. They all trace back to that one 1 cm cube, which is why the numbers are always tidy thousands once you see the pattern.