"A Book"

"Meow"
"Mommy, you're not a kitty."
"Am I a cow?"
"No."
"A horse?"
"No."
"You're right, I am a mommy."
"What kind of mommy?"
"A human mommy."
"No, not a human."
"Not a human? Well then, what am I."
Glen thinks for a minute...then with a look of intense concentration "You're a book!"