Finding a meal would be a lot easier, not to mention cheaper, if we could simply step outside and graze. But a meal made up of grass and leaves would not only offend your taste buds, it would probably give you a pretty bad stomach ache. So why is it that adults are always pushing greens on kids when humans couldn't possibly survive on a diet of grass or leaves alone?

All plants are made, in part, of a very complex carbohydrate called cellulose. This is the material that gives plants their strength, their ability to stand tall even though they have no bones. Cellulose provides structure by surrounding each cell with a tough wall. Cellulose in foods is usually called dietary fiber. Eating fruits and vegetables, which contains relatively small amounts of cellulose, helps to keep our digestive systems healthy. Fruits and vegetables are also good sources of vitamins and other nutrients that can't easily be found elsewhere.

Unlike fruits and vegetables, most plant material is low in nutrients and extremely high in cellulose. There is simply too little digestible material in most plant tissue to sustain a human, no matter how much of it is consumed.

Animals like cattle, sheep, goats, giraffes, and camels, otherwise known as ruminants, grow fat and healthy on just such a diet. That is because they have a digestive system that is very different from our own. Most scientists describe the typical ruminant digestive tract as having four distinct stomachs -- the rumen, reticulum, omasum, and abomasum -- linked together in that order.

Even with multiple stomachs, ruminants would be unable to digest cellulose if they didn't have a lot of help. Inside the first two stomachs of a typical ruminant live billions of bacteria and protozoans. These single-celled organisms possess a chemical that ruminants (and humans) lack, a digestive enzyme called cellulase that breaks down cellulose.

In keeping with their name, ruminants typically spend six to eight hours each day ruminating -- a process that involves regurgitating boluses of previously eaten food, called cuds, rechewing them, and swallowing them again. This helps to break the food down physically into increasingly smaller pieces and also gives the microorganisms in the ruminants' stomachs ample time to further break the food down chemically.