Kangana Ranaut's latest hit, 'Tanu Weds Manu Returns', is a sequel to the first, where the plot is boy-meets-girl, falls-in-love-and-gets married. The film looks at the couple's married life, four years after they tied the knot, and begins with them telling counsellors that 'the spark' is missing from their relationship. So what exactly is this spark? Is it just increasing the number of times that a couple has sex? A recent study conducted by researchers at the Carnegie Mellon University shows that increasing the frequency of sex actually led to a drop in desire and enjoyment.