Adhesives are an easy way to keep parts together or fixed in a particular place. While there are single- and two-component glues, they generally fall into one of two classes; structural — forming covalent bonds — and pressure-sensitive glues — relying on physical bonding such as van der Waals and Coulombic interactions. Structural glues provide stronger adhesive properties, but take time to cure, whereas pressure-sensitive glues are easier and faster to apply but are weaker.