system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Does airtight automatically mean water proof? How about visa versa?

For example - were space suits made waterproof for training purposes or were they already waterproof and it just so happened to work out for underwater training?
gpt
Air molecules are smaller than water molecules so airtight is generally automatically waterproof. Think about it - if you have an airtight container (say, a bottle) and hold it underwater it stops water getting in and air getting out 

BUT... Airtight things are usually designed to keep air *in* while watertight things are often designed to keep water *out* so it's not as simple as that because something like a submarine is designed to hold out huge pressures of water, which is a much more difficult proposition than holding in normal atmospheric pressure of air. 

So airtight automatically equalling waterproof only really counts when trying to prevent air/water going the same direction and only with reasonably equal pressures.