Mia’s mother has 5 kids: Lilly, Abby, Alexa, Mila, and.... Q: Who is last? A: Mia.
Knock, knock. Who’s there? Little old lady. Little old lady who? Little old lady, you don’t need to yodel about it!
Hey, dummy! The first part tricks you into naming the fifth kid Mia because you're supposed to think it's a list! The second part is a play on words, "Little old lady who?" sounds like "little old lady whoo!" and yodeling goes "whoo!" You're probably too busy eating tide pods to get it.