system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come when I stretch, I'm not able to hear things as well?
gpt
There is a tube (Eustachian tube) connecting the inside of your ear and your nose. Its purpose is to drain excess fluid and to relieve pressure in the ear. Normally the tube is held closed. When you swallow, yawn, or stretch your muscles, the tube is temporarily opened allowing pressure to equalize. An example of a time this is necessary is on an airplane when your ears pop due to the pressure change, and you need to swallow or chew gum to fix it.

Normally your hear noises around you by the sound traveling through air into your ear and hitting the eardrum. While the tube is held open, it messes up your hearing because you're getting extra sounds coming up through your nose. As well, the movement of the eardrum is affected by this extra opening. The result is that it's hard to hear noises around you. 

But for noises coming from within (your own voice, breathing, etc.) it's a different story. Noises from within do not travel through the air; they travel through your bones going straight to your inner ear. These noises are not affected by the tube being open, so you hear them louder than you normally do.