Spring pin

Okay kiddo, imagine you have a bunch of paper cups stacked on top of each other. If you want to keep them all together, you might use a special kind of pin called a spring pin. A spring pin is like a tiny tube that's open at both ends, and it's made of really strong metal. You push it into the cups so that it goes all the way through the stacked cups, and then you slide a little tool into the ends of the spring pin to make it expand and grip the cups tightly together. It's kind of like when you squeeze your fingers together to make a grip, but the spring pin does it automatically! Spring pins are used in all sorts of things, not just paper cups - they're really useful for holding things together when you don't want to use screws or glue.