The main ingredient in common super glue is a cyanoacrylate. The liquid can remain usable in its bottle because the formulation suppresses premature reaction. Once it is spread into a thin film on a surface, trace moisture and electron-donating substances initiate rapid anionic polymerisation. Many small molecules join into long chains, turning the liquid into a rigid polymer network. This is not simply “drying” as a solvent slowly evaporates.
Skin is particularly easy to bond because its surface always contains a little moisture, and components of sweat can also encourage the reaction. Soft skin lets the liquid spread into fine surface texture, increasing the true contact area. The film between two fingers is usually thin, and reaction heat can be concentrated, so bonding may occur within seconds. A thick pool of glue, by contrast, may cure more slowly inside because it is not all equally close to a surface that triggers the reaction.
Polymerisation releases heat, so super glue should not be tested on skin. Liquid cyanoacrylate should also be kept away from absorbent cotton or wool, whose large surface area can make the reaction heat rapidly. If skin is accidentally bonded, do not pull it apart; follow the product label or advice from a local poisons information service. The central chemistry is simple: super glue bonds skin readily because skin provides the conditions that make it solidify quickly.
https://pubs.acs.org/doi/10.1021/acsami.8b10717
https://www.poison.org/articles/super-glue
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