Firstborn: "Why'd they put Z at the end of the alphabet, anyway?"
Me: "Seemed like a good place for it, and something had to go there."
Firstborn: "If there wasn't a letter at the end of the alphabet, there would be no alphabet. If there wasn't a letter at the beginning of the alphabet, there would be no alphabet."
Me: "Or maybe it would never end. Maybe it would be like, abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzhelpmeIcan'tstop."
Firstborn: (Laughs.) "Maybe helpmeIcan'tstop could be a letter." (Pauses.) "I will draw it."
Updated: Behold! The letter helpmeIcan'tstop: