How we "Lose Weight"

First, forgive me if this has been discussed / posted elsewhere. I didn't really know what to search for to find a discussion.

So.... for all the different viewpoints on ways to lose weight - different "diets", exercise regimes, etc., there is a plethora of information out there about ways to lose weight.

One thing I have been very curious about but have never heard a definitive answer on:

HOW do we lose weight, as in - what happens to the weight?

Do we pee it out? Not necessarily.... For instance, let's say I'm on a weight reduction program, eating fewer calories than I burn. So one night, I weigh myself before I go to bed. The next morning, I weigh myself and find that I've lost over 4 lbs in less than 8 hours. My question is: how did my body "release" the weight?

In this scenario, I know that I didn't pee/poop between the two weigh-ins, and I certainly don't think I sweated four pounds into my mattress.

When body fat becomes metabolized - where does it go?

This is just something I've been curious about for awhile and wanted to see if anyone knew the answer.

Have a great Friday!