Cohesion comes from the same root word that "adhesion" comesfrom. It’s a word about sticking. When somethingadheres to something else (when it’s adhesive, inother words) it’s a one-sided, external thing: something (like glue)is sticking one thing to another. Things that are cohesive, on theother hand, naturally stick to each other because they are of like kind, orbecause they fit so well together. Duct tape adheres to thingsbecause it’s sticky, not because it necessarily has anything incommon with them. But two lumps of clay will cohere when you putthem together, and matched, well-machined parts sometimes seem to coherebecause the fit is so precise. Adhesion is one thing sticking toanother; cohesion is a mutual relationship, with two thingssticking together.