stupid airlock question

I'm assuming since I'm asking a stupid airlock question, I'll get a stupid airlock answer...

Maybe this is a physics thing, but I just don't fee like thinking through this...why is it that after I've affixed an airlock to a primary fermenter or (in my most recent case) a starter, the water in the airlock seems to be sucked towards the wort, i.e. the water level in the vessel attached to the tube coming out of the fermenter rises (down tube), and the water level in the vessel attached to the gas-release tube (up tube) lowers?

Talk about stupid question, how about confusing wording!?! Thanks for your help.

No stupid questions exist.

It's probably due to temperature differences. If your wort is warmer than the air around it, it will cool. As it does so, the pressure inside your fermenter falls. That causes it to suck air into the fermenter. If you have an airlock, it sucks water instead.

rewsters right, an example is When your canning jams, the lids and glass containers are heated so that when you put on the lid it creates a vacumm

Same question in another thread.

Prior to putting the airlock on let the batch sit for a few minutes so the outside and inside temps come closer together. In the meantime, place a folded paper towel over the top of the airlock (no water, no cap, no bobbing piece) and keep it in place with a rubber band. The point here is to keep nasties outside.

I've had that happen to me. If everythings sanitized, there isn't much to worry about. I put starsan solution in mine. No worries if a litle gets sucked in. If the airlock gets a little low I top it off with vodka.