and that a lot of people flirt while denying they do it

Or without even realising. Several years ago an acquaintance on whom I was developing a crush told me she was aware of this; this puzzled me since I thought I hadn't yet initiated anything like flirting, so I asked how she knew. Then she took my hand and replicated the way in which, a few days before, I had passed her some small object (probably a pen). I didn't realise I was doing it at the time, but in that casual gesture I was prolonging the physical contact a lot more than necessary, and once put on the receiving side it was bloody obvious what was going on.