My mom asked me that recently, and for me the answer is "Nothing, really." I was quite young; two or three, maybe. Now my son appears to be following in my footsteps.
At age two and a half, he's already stopping to point to letters and numbers on signs or cars or anywhere they appear, and reading them aloud. So far it's mostly just the individual characters ("F! O! R! D!"), but in at least two cases he's remembered what the whole word is. (Those two cases, in typical negative two-year-old fashion: "No" and "Stop". Well, they're both on signs on lot.) We iz proud parentz!
In not exactly related news...
Analyzing my webpage hit statistics shows clearly that the old parodies are really what people still come to read, both the Lord of the Rings film ones and the two Harry Potter book ones. This indicates to me that the smart thing to do, self-advertising-wise, is parody the first five HP books too. So, eventually I'll get started on that. Anyone have an extra copy of Sorcerer's Stone they want to send me? I borrowed the first three books when I read them.
Maybe I'll even get the first one done before the next film comes out. Which, at this rate, gives me plenty of time.