What discussion needs.....to be in Today's top.......?

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I'm really confused, what we need in our discussion to see it Today's top. *Even discussions with 4-5 responces can also be observed there, so number of responces is not criteria. *Many times topic of discussion don't has much importance, as I've observed, some useless discussions also enjoy place there. *It's not + rate to discussion too. (as per my observation) Then which r the criterias which decide Today's top discussions...?