Liquefaction of gases is like when a gas changes and ends up as a liquid. Something that helps this to happen is when you make the gas colder or put a lot of pressure on it. It's kind of like when you squeeze a water balloon and the water comes out, it happens the same way with gases! In the same way that the water in the balloon was squeezed and moved around, the gas particles are pushed around and closer together. When the gas particles are pushed together, they can turn into a liquid.