What is that tail doing back there?

Calvin sometimes thinks that his long tail doesn't belong to him. He was sitting on the floor in the living room this evening and he kept jerking his head and looking around behind him. He would catch sight of his tail and spin around and try to grab it. It was like he was thinking, "Hey, what are you doing back there?" It was so funny.

When we first got him, as a small kitten, his tail was so long for the rest of him. It looked like it was already its full size. He finally grew into it. Sometimes when he is lying on the ironing board, like in the picture below, he will reach down and try to grab his tail which is hanging down on the other side.