A boy and a girl run around on the grass at the park. The boy tackles the girl. The girl laughs. She gets up and runs away. She loves to run. He chases, she turns and they grab each other, tumble and land in a pile, giggling. After a few minutes, he tackles her again and she lands a bit hard. She is bigger and physical, but he more than holds his own in roughhousing. She pauses for a second. Then she laughs again; she’s still having fun.

Dad gets his attention, and says, “If she’s not having fun, you have to stop.”

He is two. He needs to hear this now, and so does she. And again, and again, and again, so that like wearing a helmet on the bike it is ingrained.