Elements

The wee man is developing a fixation of the period table of the elements.  It makes total sense too.  He loves order and detail, so categorising the elements in different ways is something he was bound to enjoy.  He started off by discussing something from school so I indulged him in a period table of the elements Top Trump game.  That evolved into a pop-up book and now the ordering of elements by value, freezing point, value etc.  He's even got his sister in on the act too.