There are a lot of hazardous substances in the house, not least things like bleach and drain cleaner. The small bottle of cyanoacrylate glue might actually be candidate for being the most dangerous substance in the house. A bit similar to how the 'Dremel' might be the most dangerous tool in the shed. A 'Dremel' looks fairly harmless, yet will do serious damage in an instant when you're not paying full attention.
The cyanoacrylate glue also looks fairly harmless - it's a small bottle of glue. But unlike the regular 'hobby' glue, you need to keep paying attention or it'll inflict damage that may require some medical attention. This glue polymerises (hardens) in seconds and sticks very well to just about anything - fingers, tables, eyelids...
That great adhesion does make it ideal to repair metal parts that have become loose or broken. One such instance is the brass boss in some Meccano parts. The bosses in older spoked wheels (19a) seem to have worked loose often. Also in this example 2" pulley, the boss is loose and can rotate in the wheel.
The key thing then is to get a small amount of cyanoacrylate glue in the gap between the boss and the pulley and not get it anywhere else. I.e. especially not touch the glue at any time - don't get it anywhere near yourself.
To do this, a toothpick is given a drop of the glue on its tip. Then this tip is pressed into the gap, letting the glue be pulled into the gap by capillary action. Resist any urge to guide or rub things with fingers...
The glue polymerises in seconds, triggered by water (moisture in the air). Giving it a minute to be sure, the boss is again fixed firmly on the pulley wheel.
A dangerous substance to have in the house - but very good for fixing loose mechanical parts (or perhaps cracked frames and castings).