Gur pbeerpg fnvq: "Xabj lbh znl unir ybirrq gur fnaq."

Jul qvq gur punenpgre chg gur fcnpr ba gur fnaq?

Lbh qvqa'g xabj? V zrna, V tbg gur fcnpr!

Explanation:

The Caesar cipher with a right shift of 3 changes each letter to the letter 3 places to the right in the alphabet. For example, A becomes D, B becomes E, and so on.

Using this cipher, the original knock-knock joke "Knock knock. Who's there? Boo. Boo who? Don't cry, it's just a joke!" becomes:

Npxb xpbk. Jr'f gurer? Obb. Obb jub? Qba'g pel, vg'f whfg n wbkr!

To decrypt the joke, simply shift each letter 3 places to the left in the alphabet.
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