To fully know whether something is a plant, a fungus, or something else, you must look at the cells, says Stager.

He said categorizing can be hard, “What seems like a simple question that should be easy to classify and have this simple little list is actually way more complicated.”

Biologists classify plants separately from other living things, but if you get “really nerdy” about it, you find inconsistencies. Indian Pipe grows as little white stalks. By looking at them you would think they are a fungus, but they are actually a plant, because the part that looks like a pipe bowl is a flower. Fungi don’t have flowers; they make spores instead.

If you look at a plant cells under a microscope, they look block-like,  with mush inside them. Whereas fungus cells look like fibers, as if you were to look at your jeans through a microscope, Stager said. The cells would give it away under a microscope, and the flowers would give it away to the naked eye.

Lichen, however, has the body of a fungus, and under a microscope it would look “cottony” with those little fibers. However, looking carefully you would see that the greenish color in green lichen is actually little algae cells. It’s a combination of the two, because without either you don’t get a lichen.

Algae traps sunlight like a plant, but a biologist like Stager would say, “they’re single celled. If you’re single celled you can’t be a plant, even if you make your living as a plant does.” The weird thing, Stager said, is that some algae are the size of trees. Seaweed is now classified as algae, due to how the cells are built. Stager tells his students that seaweed and algae are in a sort of grab bag, where everything that didn’t fit in the other kingdoms go.