When you hand someone a pair of scissors, you turn them round and hold the closed blades, so the other person takes the handles. Nobody is really taught it as a rule; children pick it up from being handed things. It is about the smallest courtesy there is, and it has the shape of most of the larger ones: for a moment the giver holds the sharp part, so the person taking never has to. Watch for it and it is everywhere, in kitchens and classrooms and sewing rooms, people quietly holding the part that could cut for two seconds and not mentioning it.