* Density is mass per unit volume: Density (ρ) is calculated as: ρ = m/V, where 'm' is mass and 'V' is volume.

* Compression reduces volume: When you compress a gas, you're forcing the same amount of gas molecules into a smaller space. This means the volume (V) decreases.

* Mass remains constant: The mass of the gas molecules themselves doesn't change during compression.

* Density increases: Since the volume decreases while the mass stays the same, the density (ρ = m/V) must increase.

Think of it this way: Imagine you have a balloon filled with air. When you squeeze the balloon, you're compressing the air inside. The same amount of air molecules are now crammed into a smaller space, making the air inside denser.