Nursing On Mars: It's Your Only Option

"Mommy, why is Earth the only planet with water?"
Mommy gives some dismal answer about how we are just the right distance from the sun, not too hot, not too cold, blah, blah.
Anna thinks.

"Mommy, is Earth the only planet with milk, or do other planets have milk, too?"
I think just us, A. I think just us.