I wrote a song about a tortilla.

This joke unwraps its humor through clever wordplay, specifically a pun that hinges on the double meaning of the word "wrap." The setup leads you to think about a song related to a tortilla, which is a thin, flatbread often used to hold fillings.

The punchline then twists this expectation by replacing "song" with "wrap," playing on two distinct meanings. A "wrap" can refer to a food item similar to a tortilla, where ingredients are folded inside. More importantly for the joke, "rap" (a homophone of wrap) is a popular genre of music characterized by rhythmic spoken words. The humor comes from the sudden shift in meaning, connecting the food item to a musical style through a shared sound.

Essentially, the joke tricks your brain into anticipating one thing (a song about a tortilla) and then delivers a surprise by linking it to another (rap music) via a linguistic shortcut. It's a playful linguistic somersault that highlights the versatility of the English language.