There is a difference between adhesion at the molecular level and adhesion in engineering. There is no doubt that molecules of solid materials stick together and can be separated mechanically. The problem is explaining the connection between molecular attractions and mechanical measurements. False ideas such as keying and gluing require critical assessment because they confuse molecules and mechanics. Mechanisms such as adhesive hysteresis, stringing, and clustering deserve evaluation. A rational theory of these phenomena should be based on the theoretical concept of reversible work of adhesion and on the measured quantity of adhesive energy, which includes the extra energy required to restructure the interface as surfaces move.