My DS, 3, has started doing this after witnessing a very naughty boy doing it at preschool (and quite possibly getting away with it). All attempts to confront this behaviour have so far been unsuccessful. Fortunately he is only doing it with family members but already Mum and DD have sore eyes as a result and I shudder to think what might happen, having a friend who lost an eye when playing with her sister age 9.