Yesterday at the dinner table my son says "Hey dad, did you know 111,111,111 x 111,111,111 = 12,345,678,987,654,321?"*He thought this was awesome. I've previously told him the trick about multiplying by 9 (2x9 = 18, 1+8=9, 3x9=27, 2+7=9 and so on up the times table) and a couple of other fun facts.I'd like to present him with some other neat numerical properties and math facts. He'd need to be able to grasp the concept, so something like the non-palindromic properties of 196 might be a bit beyond him. He's in (the equivalent of) 2nd grade at a Montessori school and math is his favorite subject. He's currently doing 2-digit division and he's pretty much mastered multiplication. I'm not afraid to go a bit beyond what he's learning but I don't want to go too far beyond it.What can you tell him about other interesting numbers and how to get to those numbers?Calculator tricks might be fun as well. I'm sure if I teach him the S8008 trick he'll be the King of the Playground. Also tricks that require props such as playing cards, Legos, or anything else are welcome, as long as they demonstrate the type of thing I'm looking for.I'll take book suggestions as well. Christmas is coming...*interestingly, the first Google hit for 12345678987654321 is a MeFi thread.