if an organism eats something then that something is a source of food to that organism & herbivores only eat plants ->  plants are a source of food to herbivores  plants are a source of food to herbivores & a mouse is a kind of herbivore ->  plants are a source of food to mice  plants are a source of food to mice & a mouse is a kind of animal & food is a source of energy for animals ->  plants are a source of energy to mice  plants are a source of energy to mice & one can get something from a source -> mice get energy from plants