How do pagers (beepers) work?

How do pagers (beepers) work? Yes, there was wearable wireless communication long before cellphones and smartphones. And you know what? It was awesome!

Your cellphone gets in the way of your daily life when everyone can just call you and you have to stop what you're doing to talk to them. With pagers, people would page you and then patiently wait for you to return their call. If you didn't call back after a half hour or so, they would assume you didn't get the page and call you again. Very simple, and civilized. Unlike today when people expect to reach you instantly, at their convenience and not yours.

So, since pagers are slowly being phased out as more and more paging transmitters are being taken off the air to cut costs as the customer base dwindles, I thought it would be a good idea to preserve a bit of communications history before it's too late and do a simple demonstration.