Could they make some sort of drink that allows humans to eat grass & leaves?

We can eat grass/leaves. We just can't break it down if digested whole. Put it in the blender and we can digest it.

We could probably take certin things out of the plant to make it digestible, and than have a rose smoothie!

Certain people do mascerate tender shoots of Wheat,Jowar, Peral millet into a fine paste along with water & consume it daily. It appears to give them vigour & keep them healthy.Such a practice is already in vogue in India.

Taking a cellulase supplement would allow us to digest cellulose for a short time (if you ate the cellulase at the same time as the cellulose).  Why don't we have this?  We might, but why would anyone want it?  Cellulose provides us with fiber, and fiber is very good for you for a variety of reasons - regularity, removes cholesterol and toxins from your body / colon.  

Blending up the grass / leaves would NOT allow us to digest it as someone stated in your answers.  It would be a good source of vitamins and fiber though. Grass juice is quite good for you!  Just don't eat off a dirty lawn where chemicals are sprayed, dogs crap, or other dirtiness is present.

Humans can eat non poisonous  grass and leaves as the acid, bacteria, and enzymes in human saliva is more than sufficient to break down cellulose to levels where the digestive system can effectively extract the nutrients from these substances and expel the unused portion as waste.

The reason many people have difficulty fully digesting leaves and grasses mainly stems from insufficient time spent CHEWING and, NO - unless you unload a healthy dose of spit into the mix - your blender, while it does increase the surface area of your food - like chewing - it does absolutely nothing to begin the chemical process of digestion.

Anyone who juices is well aware of the accompanying runny - poo - poo -syndrome.

That said, even with a heapin' helpin' of saliva and 10 full days of chewing, our single chamber stomach is incapable of digesting certain parts of many grasses, to a degree where these foods can even begin to approach "safe for human consumption!"

Sad thing:  Most people consume a half pound or so of this toxic substance each day and are completely unaware of the damage it causes.

Sorry to be rude.  I was headed onto an entirely different topic that I won't bore you with here.

Simple answer.  Yes you can eat many leaves and grasses - as long as you stay away from the mature grasses you should be OK.

Be sure to check with your local library, nature guide, or herbalist to see which are edible and chew really really good.  

Bonappétit!

Quintin

"That which does not kill me only makes me sick."

We eat grasses and leaves all the time. Think about it! What is a salad really but grass and leaves?

You are thinking of animals like ruminants, who get all of their nutritional value from processing only leaves and grass, and so have a different form of digestion that involves symbiotic bacteria to process that material more efficiently.

We are omnivores though, which means we are capable of breaking down both animal and plant material for nutrition. Sure, it is more difficult for us to break down the cell walls/cellulose of plant material, but we just use it at the end of the line to make our poop more...poopy.
 
But we have some good grindy teeth in the back of our mouths to help squish out a lot of the other important nutrients in plants and also a long enough digestive tract to slowly process all those greens before they reach that point in our digestion.