I've got a few airlocks and Demi-johns passed down from my dad.
The Airlocks though... look different to most, I've done a bit of research and they appear to be Round Cylinder Airlocks. I'm not doubting they work, I just can't quite see how they do, can anyone explain?

There are two parts to them, the part with the bung on and another part that acts as a lid...