system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: What is the phantom sensation of a phone ringing in your pocket when it's not even there?
gpt
Your muscles twitch a little bit, randomly. Bundles of muscle fibers will contract a little bit, just to keep the blood circulating. Normally, these movements are too small for you to notice. You have a lot of strong muscles in your legs, so if you sit still for long enough, sometimes the random movements are large enough to feel if you're paying close attention. 

You can learn to pay attention to the sensation of vibration in your legs. This is what happens over time when you keep your phone in your pocket and check it whenever it vibrates - you've rewarded yourself with interesting content/social connection for checking that vibration. So you learn to pay attention to vibrations in your leg. 

Then your muscles contract randomly, and it vibrates the muscle. So you go to check it.