I used to live in a house with a big back yard and lots of garden. One day in summer I was digging weeds with my back to the house at the far end of the yard. My toddler son was inside with his dad, my baby daughter was napping. It was a quiet afternoon. After a while I heard little feet thudding on the grass - my son running across the lawn towards me. I turned around expecting him to come up and jump on me. There was no one there.