I suppose my daughter's first obsession was books. At around 1 year old she would wake us up not by yelling "Mommy" or "Daddy", but by yelling "Book!"

Somewhere around 18 months of age the picture book dictionary became her favorite book. She seems to want to know every word in the English language. So you could say she is obsessed with words.

Currently at 29 months, she's pretty interested in the first moon landing (Apollo 11). There's a page in her children's encyclopedia devoted to it, which she makes me read quite often. I recently thought to show her a video of it, and got confirmation that she's been paying attention. She pointed to the screen and said "There's the lunar module."

Other topics of interest include bones and birds. In a couple weeks I'm going to take her to a zoo and she'll see an ostrich. She's pretty excited for that.