The idea came from a line in another book by Henkes, "The cat thought the moon was a bowl of milk." Henkes gradually expanded on that for Kitten's First Full Moon.
The concept for "Kitten's First Full Moon" stemmed from a similar idea found in another book by Henkes, "The Cat Thought the Moon Was a Bowl of Milk." This initial idea was further developed into the story "Kitten's First Full Moon."