WHY HAS SHE STARTED SMACKING?

[MollyMooMin]

My DD is 17 months and has started smacking us, normally in the face. She's never seen us do it, so why? Otherwise, she's so pleasant natured.

They all do it. I bet if you did a survey of Mners 90% of them would say that their toddlers did it at some point.
They suddenly realise that they get a reaction by flapping their little hand around until it makes a nice smacking noise on someone else's skin.