Ever heard of a dude called Avogadro? Well, he came up with this super cool law stating that equal volumes of any two gases, no matter what they are, at the same temperature and pressure, contain the same number of particles. It's like saying you've got two identical balloons - one filled with helium, the other with oxygen, both inflated to the same size and in the same room (hence, same temperature and pressure) - both balloons contain the same number of gas particles! Pretty neat, huh?

Now, we've got this thing called the molar volume of an ideal gas. It's a constant value at a given temperature and pressure. For instance, at STP (Standard Temperature and Pressure - it's like the room temperature and pressure we live in), the molar volume (let's call it Vm for short) of an ideal gas is equal to 22.7 dm³ mol^–1. Imagine a box with sides of about 28 cm (1 dm is 10 cm), filled with any gas at room temperature and pressure. The gas inside is about 1 mole (Avogadro's number of particles, which is around 6.022 x 10²³)!