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 inspiration for "Kitten's First Full Moon" came from a passage in another book by Henkes, "The Cat Thought the Moon Was a Bowl of Milk." Henkes developed this concept further to create "Kitten's First Full Moon."