Have an obsession or just something that they're extremely interested in? When we were little I remember my cousin LOVED monkeys. I never really got that into one particular thing, but my daughter's 5 and in Kindergarten now, and she's all about science and especially dinosaurs lately. She'd liked them before, but now she tells me that when she grows up she wants to be a paleontologist. We went to Barnes and Noble and spent at least a good 10-15 minutes (probably more actually) sitting looking at the dinosaur books. We have one that she got at her bookfair maybe a month ago, and it's been her pick for her bedtime story more than any other book since then. Pretty sure I could tell you all about Suchomimus and Pentaceratops if you wanted me to lol. I'm not complaining by any means, I think it's great that she's so interested in this one subject, but since I've always been the kind of person who gets a little bit into everything I wanted to see how many other kids get super into one thing.