Glue does not dry, it changes state
The difference between an adhesive that evaporates and one that reacts.
Some glues harden by simply losing water to the air, leaving behind the solid material that was dissolved or suspended in it all along, while others harden through a chemical reaction that locks their own molecules together into a solid, and confusing the two, treating every glue as though it works by drying, explains most of the mistakes people make with the ones that do not.
The mechanism behind curing
An ordinary white wood glue, or a child's craft glue, is mostly water carrying a suspension of a polymer that cannot flow once the water is gone. Brushed onto a surface, the water begins evaporating immediately, and as it leaves, the remaining polymer particles are drawn closer together until they touch, fuse and set into a continuous solid film. Nothing chemical happens between the ingredients themselves, the glue simply loses the liquid that was keeping it spreadable, the same way a puddle dries into nothing on a warm pavement, except that here something solid is left behind once the liquid has gone. Epoxy and cyanoacrylate work by an entirely different route. Epoxy hardens because its resin and hardener react with each other once mixed, their molecules joining into a rigid three-dimensional network, and cyanoacrylate hardens because trace moisture triggers its own molecules to link into long chains. In both cases nothing evaporates and no mass is lost, the liquid becomes a solid purely by rearranging itself chemically, which is why both will cure perfectly well sealed inside a joint with no air movement at all, somewhere a drying glue would simply stay wet indefinitely.
The frying-an-egg comparison
Cracking an egg into a hot pan shows the same distinction from the kitchen. The clear, runny white does not become solid because water leaves it, in fact it loses very little liquid at all while frying, it turns opaque and firm because heat rearranges the proteins inside it, unfolding them and letting them link together into a solid mesh. That change cannot be undone by adding water back, since a fried egg white will never return to its raw, runny state no matter how much liquid is added to it, because the transformation was a rearrangement of the protein's own structure, not a loss of liquid that some added liquid could reverse. A curing epoxy or a set bead of superglue is in exactly this position: dissolving it in a solvent might swell or soften it, but nothing will turn it back into the liquid it started as, because what happened was a one-way chemical change, not the reversible loss of a liquid that drying represents.
One figure worth keeping in mind
A typical white wood glue is well over half water by weight before it goes on, and losing that water is most of what "drying" means for it, which is why a thick pool of it in a corner where air cannot easily reach stays soft and tacky for days, waiting for water that has nowhere to go. Cyanoacrylate and epoxy, by contrast, are applied at very close to their final cured weight already, since curing rearranges what is already there rather than removing anything, which is why they can set hard in exactly the enclosed, airless conditions that leave a drying glue permanently wet.
What this changes in practice
Knowing which kind of glue is in hand changes how it should be used. A drying glue needs air movement and thin application to lose its water quickly, and clamping two non-porous surfaces tightly together with it can trap the water indefinitely, leaving a joint that never fully sets. A reacting glue needs the opposite treatment in one important respect, since sealing it tightly into a joint does it no harm at all and can even help, but it does need its two parts mixed in the right proportion or, for cyanoacrylate, enough ambient moisture nearby to trigger the reaction in the first place, since without that trigger there is no chemistry to start the change regardless of how much time is allowed to pass.