I was just having this discussion on the phone last night. I don't think that you should laugh.
I have thought about it and realized that everything really is part of everything else in one huge, dynamic system. Taking it down to the atomic level, the molecules that are a part of your body right now are comprised of atoms that were at one time part of something else. Most likely, your anabolic processes built these molecules from atoms that you ate (or drank) or breathed in.
For instance, that hamburger you had for lunch: the bread is made of molecules that used to belong to a wheat plant, which in turn got its constituent atoms from the air and soil, perhaps fertilizer. The nutrients in the soil came from either the air (nitrogen fixation) or from decaying organic matter (things that used to be plants or animals). The meat in the hamburger came from a cow, which is an animal just like people. They get their chemical makeup from feed and air. They turn the nutrients in grass and water (and in England's case, the ground up remains of other cows) into meat that you consume. Some of the water you were drinking might have once been a part of the products of combustion of a tree that went into the atmosphere and then condensed in the source of your water.
Now that we have the incoming...time for the outgoing. You are constantly shedding skin cells which then break down and become part of something else. There are bugs that eat your poo. They continue the cycle...you get the idea. I expel methane gas into the air without ever consuming methane.
Even outside of animals...minerals are constantly transported by water, undergoing processes of erosion and solidification. Volcanic ash becomes sediment in rivers and part of the soil that ends up growing plants.
I wish I had a better way of articulating this idea.