Photosynthesis is a process that a plant uses to make oxygen. The plant absorbs light from the sun, it takes in water through it's stem, and it collects carbon that animals breathe out, so the plant can make oxygen for animals to breathe in. The oxygen is poisonous to the plant, but the carbon is good for it. But the oxygen is good for the animals, but the carbon is bad for them. So you could say that by getting rid of what is bad for them, they are giving something that the other needs, and the other gets rid of what they need, to give another what they need. It is a constant back and fourth cycle.