Question

How does the carbon in grass become part of a lion?

Answer 1

The carbon ends up in the food the lion eats.

Answer 2

Usually because they are in the same food chain. for example, lions are carnivores (eat other animals). it is likely an animal lions eat are herbivores (eats plants like grass) and therefore, when the lion eats the herbivore, it receives the grass and plants that herbivore ate as well.