Middle Daughter started with an X in one corner. The Boy promptly countered by marking an X in another corner.
No, I'm X, said Middle Daughter.
Oh, sorry, exclaimed The Boy, then carefully traced over his X with another X.
Not X. You're O. I'm X.
OK, said The Boy, then again traced over his X with...another X.
What are you doing? said Middle Daughter.
The Boy thought for a second. I'm discovering that's it's really hard to cross out an X.