Rating and Ranking System

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Has anyone figured out yet how the Interesting, Top Rated and Hot Post are chosen. When you look it seems like Rated and Hot are the same. You can see the most recent is all the new topics. Is there some sort of grading or rating system they use to classify posts? Just curious. Because on Hot topics it seems like some of them don't have many comments or even likes on them. So how do they determine they are Hot or Not.