What is the smallest living thing?
The smallest living things are viruses. They are like car engines without any wheels or bodywork attached, they have no way of moving around and no cell wall, unlike other living thing. Viruses do not even have any way of reproducing themselves. In order to produce more viruses, they have to invade other cells and take them over. This is why viruses cause diseases. When you catch a cold, the cells in your nose have been taken over by viruses so that they can reproduced themselves.
How to live on leaves
Animal such as deer and horses have bacteria in their digestive systems that allow them to digest grass and leaves. Our ancestors once had these bacteria too, and attached to your intestine is an 'appendix'. This empty pocket is all that remains of the large structure which aonce held leaf-digesting bacteria.