How does my message know which phone to go to?

Picture a grown-up tapping send on a phone. Whoosh, the little message flies away and the screen goes quiet. Listen closely next time, and you might hear that soft swish too.

A moment later, Grandma's phone buzzes far away across town. Her neighbour's phone stays quiet, and so do all the others. So how does your message know which phone to go to?

Every phone has its own phone number, and no two phones share the same one. When you choose Grandma's name, your phone tucks her number onto the message. The message itself does not know anything, but it carries that number like an address on an envelope.

Next, your phone sends the message out as radio waves, which are invisible waves that zoom through the air. Think of ripples spreading across a pond, only nobody can see these ones. Radio waves let the message travel without any wire at all.

The radio waves fly to the nearest cell tower, which is a tall pole that catches and sends phone messages. You may have spotted one beside a road, standing like a metal tree. The cell tower catches your message so it can pass it along.

From the cell tower, your message races along cables to the phone company's computers. Those computers read Grandma's number on the message. But Grandma might be at home, at the shops, or riding a bus. How do you think the computers can tell where her phone is right now?

Here is the secret. Every phone that is switched on quietly says hello to the cell tower closest to it, again and again. The phone company's computers keep a list of which phone is near which tower, so they know Grandma's tower.

So the computers send your message along cables to the cell tower closest to Grandma. Your message zips through them in a blink.

That cell tower sends the message out as radio waves again, all around it. Lots of phones nearby catch those radio waves. Each phone checks the number on the message it catches.

Grandma's phone spots its own number and keeps your message, while every other phone lets it pass. Buzz! Her screen lights up with your words, and the whole trip took about one second.

Radio waves, cell tower. Your phone puts the phone number on your message, and the phone company's computers send it to the cell tower near the phone with that number.

Next time a message flies away, you can picture its whole trip to the right phone, so rest well now.