Curious Robin is ... Curious

I have a bird feeder attached to the outside of my window. It mostly attracts the tits (blue, great, coal) and sparrows. All of these normally either grab food an fly to the trees, or stay, but with their back to the window, so they can keep an eye out for danger.
However, a couple of days ago, a robin turned up, and unlike the others, he seemed very interested in what was going on inside. I could actually see the eyes follow me as I moved around the room.