My friend Zena has twin boys, aged about two. Last christmas a few people were round at her house for mince pies and enjoyed the wonderful sight of Ash and Kim diligently taking all the decorations off the tree and putting them back on in a big shiny cluster about two feet off the ground. Then Zena would take a few and put them back, and the twins would get them off again and put them back where they wanted them. It went on like this all day, they never seemed to get tired of it :)