Rocks

L: Mom, there's a rock in my butt
B: what?
L: Did you know there's a rock in my butt?
B: Are you sure? (Mind going crazy with how a rock got in her butt)
L: Yup, it's right in here (pointing to her crack)
B: Oh!! (Inside my head: thank God I do not have to take her to the ER to get a rock out if her butt).
That's your tailbone, it's the bottom of your spine.