18? or 100?

Christa was taking Lexi to the doctor today for her five-year-old checkup - by the way HOW IN THE HECK DID SHE GET TO BE FIVE????? - and they were doing "math" on the way there. By this I mean Lexi was giving Christa addition problems and then correcting her. My favorite is this one: Mom, what's 9 + 9? Of course, Christa answered "18." Lexi's response? No it's not. You told me it was 100 when I was three.

You can't make up this stuff.