Knock, knock!

Who’s There?

Ash.

Ash who?

Bless you!

Explanation: Ash who?  Sounds like “Achoo!”   “Achoo” is how the sound of a sneeze is written in English in the United States.  So when the asker asks “Ash who?” it sounds like Achoo!  And if someone sneezes you say “Bless you!”