My cat, Mystery, is napping on my bed. I joined him for a bit to snooze as well. He was resting his chin on my hand. Once I woke up to go to the bathroom I slowly moved my hand out from under his chin and noticed it was slightly wet. My kitty was drooling in his sleep. My primitive research showed that cats drooling in sleep is a sign of a good rest and a happy cat, similar to humans drooling while sleeping.
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