There are many things that the three have in common, including being made of matter, being alive, having a large percentage of their DNA in common, etc., but the thing that really narrows down the link between these three organisms is a substance called chitin. Chitin is made up of long chains of sugar, bonded with nitrogen, to form a tough, flexible substance. It makes up the cell walls of mushrooms, the exoskeletons of arthropods such as grasshoppers, and the sharp beak of cephalopods like the squid.