...is brass,
Mommy's gonna buy you a looking glass.
If that looking-glass gets broke,
Mommy's gonna buy you a billy-goat.
If that billy-goat won't pull,
Mommy's gonna buy you a cart and bull.
If that cart and bull turn over,
Mommy's gonna buy you a dog named Rover.
If that dog named Rover won't bark,
Mommy's gonna buy you a horse and cart.
If that horse and cart fall down,
You'll still be the sweetest little baby in town.

At least, that's the way I learned it; I'm sure there are endless variations.