After seeing the Blue's Clue's episode about space ("Well the sun's a hot star...Mercury's hot too..."), my 3 year old asked a ton of questions about space. I think that it would have been somewhat easy to blow him off, but we made the extra effort to find answers to his questions, buy him relevant books, and show him things on the Internet. It became his obsession for a long time. He knew the planets, many of their moons, the principle of gravity, which planets were "the Gas Giants" etc, simply because we followed his lead. I never had much of an interest in space, but my son helped me discover how interesting it really was. One day, when he was 4, he corrected something I said about fire on the Sun (because there isn't actually true fire on the Sun). Meanwhile, at 3, he was still figuring out how to pedal a ride-toy. I would say that, in general, he was physically "behind" most kids his age.
His 5th birthday party had an engine theme, complete with pictures of pistons and spark plugs as decorations. He likes electricity. He became very interested in molecules, which led to interest in atoms...the electrons tied in with his electricity interest. We decided to buy him a chemistry curriculum and now he states, "the nucleus of the atom can be broken into quarks" (skipping protons and neutrons but still accurate). He desperately wants an electron miscroscope. But he's in the other room right now laughing hysterically at the Teletubbies. He seems very innocent and unworldly for his age. He thinks Barney is a fabulous show (but also Monster Garage and mechanical shows). I'm sure that some of his interests are much more child-like than most boys his age.
He doesn't read, but his reading skills appear to be average for his age right now. He has almost zero interest in animal things. Animals seem to be a popular thing to teach in early elementary and he would have very minimal interest in that. He still doesn't know what a gun is (it just hasn't come up yet) and I have to tip-toe around death topics. He is much less capable, emotionally, of dealing with violence and scary themes that most boys his age. He hid in the kitchen the other day because something on Max & Ruby upset him. I have to be very discriminating in what we read to him, because he gets upset by anything "scary".
My youngest is more balanced and has not expressed unusual interests yet. He likes trains, which is a pretty common interest for his age. He does like geography and will use Google Earth by himself (just randomly clicking and zooming). I'm not around a lot of kids his age, so I'm not sure what's ahead or behind with him. Edit: I just thought of one. He loves musical instruments and will sit for a very long time watching people play them. When I take the kids to children's classical concerts, my oldest typically loses interest within 10 minutes and my youngest will happily sit for well over 30 minutes with rapt attention; I'm sure he could sit for an hour. So, to follow up on that, I'm looking for concert opportunities for him (and will probably leave ds1 with Daddy). I taped a "Reading Rainbow" episode for him that was about playing musical interests.
I just know that my oldest has some odd interests and I'm glad that we followed his lead. He's such a happy guy. I've been learning things with him and rediscovering topics that were "boring" in school. I'm pleased that we're able and willing to follow his lead and wander all over the place. I'm homeschooling myself too!