Not An Earthworm

A father had promised his two young sons he would take them on a fishing
trip. The boys were digging for fishing bait in their parents' garden.
Uncovering a many-legged creature, one of the boys proudly dangled it
before his Father.

"No, son, he won't do for bait," his Father said. "He's not an
earthworm."

"He's not?" the boy asked, his eyes wide. "What planet is he from?"