system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How does Bluetooth work?
gpt
Each Bluetooth device has a little radio in it. The Bluetooth radios can talk to each other if they are on the same channel. It's kinda like setting the TV to the right channel so you can watch a particular show. 
TVs stay in the same channel until someone changes the channel. But Bluetooth radios don't stay on one channel. In fact, they jump around about 80 different channels all the time! All that jumping around follows a pattern, and there are many different patterns. 
The only way two Bluetooth radios can talk to each other is if they both jump between channels at the same time following the same pattern. But if the first Bluetooth radio doesn't know the pattern or timing of the second Bluetooth radio, how do they find each other in the first place? 
When you want to Bluetooth products to talk to each other, you first have to 'pair' the Bluetooth radios. When the radio is put in pairing mode, it goes to a specific pattern that all Bluetooth radios know. That way, the two radios can find each other. Once they find each other, and after they ask each other some questions to make sure they're talking to the right radio, one radio tells the other radio what pattern it is going to use, and provides the timing. Then they both jump to the pattern and talk to each other as they jump from channel to channel. Usually, each radio stores the codes so they can find each other again next time (a process called bonding).
Once they are paired and connected, the two radios can exchange all sorts of data. That data could be music streaming from your phone to your headphones, letters and words from your keyboard to your computer, or movement data your smartwatch to your phone.