kcab--I have a good friend (now in his mid-40's) who was telling me that when he was in elementary school he was fascinated by the city bus system. He memorized routes and if a bus went by that didn't seem to be in the right place at the right time, he would call the transit office to ask about it
I don't know if this qualifies as a hobby, but when my DD was 6 she started a screw collection. I thought she had forgotten about it (I only see it when we are sifting through the wreckage to try and clean her room), but out of the blue the other day she turned to me and said, "I need some new screws for my collection"!
My DS focuses on more "conventional" interests, but he dives into them at a level that drives everyone crazy. If I never see or hear the words "Lego" or "Star Wars" again, it will be just fine with me....
