Re: Why do humans eat?

Humans (and all animals) eat because they need nourishment. The 
nourishment provides calories (or energy) for things like moving around, 
thinking, and growing. In addition, food provides the stuff that we are 
made of like  amino acids to make proteins, like those in the brain and 
muscle, calcium for bones and teeth, iron for blood, minerals, salt, 
vitamins, etc.

I don't know exactly what your teacher was expecting, but I suspect that 
this is (s)he meant. I hope this helps.

The following web site provide information about nutrition:
 http://navigator.tufts.edu/ http://www.nal.usda.gov/fnic/ http://www.nutrition.gov/