Why is poop green and why do people poop

I need an answer fast! please and thank you. For biology!

It is not always green, it may be brown...
It depends on the food u eat it will be green if u eat spinach for example!
Why do we poop? since not all the food we eat are needed, meaning we eat the food, absorb the nutrients needed & the unwanted/undigested parts are secreted! ;)

i believe that when your poop is green, it's from excess iron in your diet.

also when you eat, your body sucks out all the nutrition it needs from the food and dumps the rest out through your lower intestine in the form of poo. lol. i'm pretty sure im right, haven't taken biology in a few years though.