This is a behavior called "flehmen". Animals have a secondary sense of smell that causes them to make faces when they use it. The second sense of smell is reserved for those scents that are mating season triggers.
Some were a little confused where girls pee from (it’s OK, it happens to the best of us). Extra points for “tush.” | Here's What Happens When You Ask A Bunch Of Adults To Label Male And Female Reproductive Systems