DS4 was "0 to 60" on math this year, but he's not really reading yet. (He's sounding out words, but he's not fluent with reading. In contrast, DS7 was reading chapter books by this age.) He had a similar explosion with writing 6mos. before that. Within 36 hours, he went from writing squiggles at best, to handwriting that looked like his big brother wrote it. It just seems to be his M.O. I'm suspecting that he's a visual-spatial learner, since that's the way they often do things: not necessarily early, but all at once when they do get them.
6mos. ago I wasn't sure DS4 was GT. But his math explosion tells me that he probably is. He's just differently GT.
He goes to K next year. I am awaiting a reading explosion in the next couple of years. We shall see...
