* Density is mass per unit volume.

* Mass remains constant: When you transfer the gas, you're not changing the amount of gas molecules (and therefore the mass).

* Volume changes: By putting the gas into a smaller container, you are decreasing the volume the gas occupies.

Since density is mass divided by volume, and the mass stays the same while the volume decreases, the density of the gas will increase.

Example:

Imagine you have a balloon filled with air. If you squeeze the balloon, you're reducing the volume of the air inside. Even though the amount of air (and its mass) stays the same, the air becomes denser because it's packed into a smaller space.