I'll try to give a simple explanation as I understand it.
1. U eat some food and it lands in ur stomach.
2. Ur stomach churns up the food with acid and other stuff and sends it to ur large intestine which is about 4 to 6 feet long I think.
3. In ur large intestine chemicals from ur liver and pancreas and also bacteria in ur large intestine help break down the food even more.
4. As the food makes its way thru ur large intestine, the nutrients from the food are absorbed by ur body.
5. As the food continues thru ur large intestine, it gradually begins to look like the brown stuff u call poop, *bleep*, crap, whatever. Things like corn, peas,peanuts and other nuts won't break down completely so u will see what's left of them in ur log when u take a dump.
6. The last foot or so of ur large intestine is called the colon or rectum which connects to ur butt hole. When ur rectum gets full enough that's when u will feel like u need to take a dump.
7. U pull down ur pants and set on the toilet or wherever.
8. This is the best part! U relax or maybe push a little and ur butt hole opens up and out comes a brown log. If u eat enough fiber and get enough liquids, u most likely will be able to push out 1 solid log. The more food u eat, especially if it has lots of fiber, the longer ur log is likely to be. Like that PJ dude here that eats alot of fiber and makes logs that r like a foot long or longer. :D
Hope that helps!