I have always been brewing in plastics bottles (not very professional, but for me it works without problems) and so I thought of some better way of getting CO2 out of the bottle and no oxygen in than capping the bottle and letting it squeeze when there's high pressure.
The liquid-filled airlocks are fine but making them is a horror and also, I was used to let bottles in cellar for long time until it's finished and sometimes I found out that it dried...
So there's one primitive airlock that needs no care.
It works just on two facts:
1)There's still higher pressure in the bottle than outside (this works until yeast dies or runs out of sugar)
2)Relative molecular weight of CO2 is 43.99 g/mol. Air has 25.93 g/mol. Result of this is when you have CO2 in bottle, it won't mix with air until wind blows into the bottle (at least for some time, due to diffusion).
Step 1: Materials
OK, now what will we need: