system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How do herbivore animals get enough calories in a day?

I just read that 1Kg (About 2lb) of grass has like 64 calories. 

How on earth do animals that run around all day like deer or huge animals like elephants eat enough calories from plants in nature to function? 

I know they don’t only eat grass, they eat roots, bark, sometimes fruit if found, but if sedentary humans consume something like 1200-2500 calories a day, how on earth do these animals that move a lot can get enough calories?! 
I read that for example deer burn anywhere from 6000-9000 calories a day.
gpt
The 64 calories is what's available if you ate it. Cows and the like let the grasses ferment in their stomachs which allows them to get calories from the cellulose which is otherwise indigestible.

The effective calories a cow or horse can get from a lb of hay is more like 800-900 per lb.