Rheology, is a complex subject that helps describe how adhesives adhesives can behave like a liquid to bond but a solid to debond. As adhesives continue to evolve, so to do methods for optimising rheology. How can a material behave both as a liquid and a solid? It may seem contradictory, but that is what adhesives do. They must behave like a liquid to attain intimate contact with a substrate for bonding, but like a solid to resist to debonding. This challenge is often overcome by adhesives that undergo changes in physical state between bonding and debonding. Hot melt adhesives cool from liquid to solid, while chemical changes control reactive adhesives' state. For most adhesives applied as liquids, the interfacial energy between them and their substrates provide the force driving them to wet out and attain intimate contact with the surface. The adhesive's viscosity makes it resist the deformation required for this spreading on the substrate. Lower viscosities therefore nominally allow better wetting and therefore bond formation.