Sneaky Sitter

This morning I put Sam down on his tummy in the living room floor and went out of the room.  When I came back in the room about 30 seconds later he was sitting up.  It took me a minute to process that he had been laying down.  I put him back on his tummy and watched him for 10 minutes but he didn't get into sitting again.  He seems to only want to do it when I am not around.