system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do your lips get really dry really fast after licking them?
gpt
Your lips are a mucous membrane, which involves a high level of moisture.  Water/saliva isn’t a good moisturizer.  In fact, it does the opposite.  Applying fluid actually drys a surface that is allowed weep lipids.  To moisturize you want to apply lipids, like chapstick.  Your body naturally produces oils, but your saliva washes them away and dries the lip.  This makes the skin dry out and crack instead of keeping a moist continuous layer.