Super glue is an acrylic resin (artificial gum) made from chemicals obtained from petroleum. When exposed to moisture the small, loosely-bound molecules in this resin join to form long, tight chains. This enables it to harden instantly, a process known as “polymerisation”.
Inside the tube, an acid prevents the glue from hardening. But the moment it touches any surface, moisture in the air makes the gum polymerise. You must be careful not to drop the glue on your fingers, as it bonds with skin easily.