Q: What is the math teacher’s favorite dessert?
A: Pi!

Explanation:  “Dessert” (with 2 s) is a sweet treat after a meal; “desert” (with 1 s) is an area of the earth that gets little or no rain.  An easy way to remember: people always want more dessert and it has more of the letter s.  One type of dessert is pie; I especially like blueberry pie.

Pi is a math number that helps us understand circles.  (It is also the name of the main character in The Life of Pi.)

This joke is funny because it plays with the meaning of pi/ pie which sound the same but only one really tastes good.