system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does our tongue rest at the top of our mouths and not the bottom?
gpt
It doesn't for everyone, but if it does for you, then this means you have good oral posture! Your body likely continues to maintain this posture because it has so many good benefits, while resting with your tongue down can cause a lot of problems. 

The tongue is a very special muscle because it doesn't have skeletal support (you don't have a tongue bone)- it also has the ability to move in many different ways thanks to it having four internal muscles that run in different directions- this means that your tongue is able to move and rest in a lot of different ways. Just as standing with good posture has benefits, so too does maintaining good oral posture. If you stand with your head slumped, you'll notice that your neck, back, and shoulders hurt a lot! Similarly, if you have bad oral posture you might find that it causes pain/ misalignment in the jaw, shoulders, and neck. This is because the base of your tongue doesn't just float around in your mouth- it is connected to a bone called the hyoid, just under your jaw in your neck, which is connected to your Adam's apple/ wind pipe/ vocal chords in turn by different muscles- your Adam's apple bobbing when you swallow actually helps close off your windpipe to protect your airway! Your facial and neck structures also play into your mouth posture a lot because the mouth is so important for breathing and feeding- two of our most essential functions. In fact, if you rest with your tongue down, you're likely to have a visibly 'weak' chin (which can be improved with improved posturing if it is from tongue placement <'mewing'>- sometimes though, people really do just have small jaws).    
Some of the benefits of good oral posture include: preventing dental crowding/ tooth grinding or clenching and therein decay, preventing pain in your jaw joint (TMJ), helping you breathe through your nose instead of your mouth (better for filtering air and also warming it up, which is good for your respiratory system), and maintaining the roof of your mouth (your palate) which is important for swallowing (by closing off the nose)/ helping with chewing and for speech. Your tongue should also be at the top like this when you swallow so resting with your tongue down may make your swallowing less effective unless you have other medical conditions that would make tongue down swallowing safer (like if it takes a longer time to close your windpipe).  Finally, bad tongue positioning may be associated with having more middle ear infections because a little tube runs from the back of your mouth to the space behind your eardrum- this helps your ears 'pop' after airplane rides.