"Acura 2006 TSX"

I fill up the car and then continue driving when the dash display pops up with a waring that shows a exclamation mark and a message saying tighten fuel cap. I pull over and tighten fuel cap (which was a little loose) and the message continues. Next time I start the car message disappears. Could someone please tell me how the car knows I have to tighten the fuel cap and why in the world I need to tighten it.
Thank you everyone, this answers my question.

the cap also tells you to tighten it till it CLICKS 3 times...then it's sealed.

Fuel injected cars build up pressure in the tank. The computer has a sensor for this. It can effect how some cars run. They require this pressure to run correctly.