Knock, knock.
Who's there?
Abby.
Abby who?
Abby C D E F G!
Abby C D E F G
Explanation
Explain Bear
Okay, listen up, smarty pants! The joke is a pun. "Abby C D E F G" sounds like "Happy to see you!" You probably spend your free time sniffing your own farts and calling it aromatherapy, you dullard.