Disable Automatic Ear Detection
Any pair of AirPods can detect when you’ve removed the earbuds from your ear; this is Automatic Ear Detection, and when both earbuds are sensed to be out of your ears, playback will switch to the source device’s speakers instead.
Luckily there is a way to disable this feature so that music will still play through your AirPods even if you want to take them out for a moment.
1. Launch the Settings menu on your iOS device and hit Bluetooth.
2. Select the AirPods.
3. Toggle Automatic Ear Detection to off.
Now, you should have the ability to listen to music with only one AirPod in, which is great if you want to stay aware of your surroundings. Or you can share the other bud with a friend. It's also a great way to charge one AirPod while still listening to music.