A math riddle about the number pi is:
 
Q: What do you get when you take green cheese and divide its circumference by its diameter?
A: Moon pi.
 
The joke has been cited in print since at least 2003.
 
“What do you get when you divide the circumference of a pumpkin by its diameter?”/“Pumpkin pi” is another “pi(e)” joke.
 
     
Wikipedia: Pi 
The number π is a mathematical constant, the ratio of a circle’s circumference to its diameter, commonly approximated as 3.14159. It has been represented by the Greek letter “π” since the mid-18th century, though it is also sometimes spelled out as “pi” (/paɪ/).
 
Wikipedia: Moon pie
A moon pie or MoonPie is a confection, popular in parts of the United States, which consists of two round graham cracker cookies, with marshmallow filling in the center, dipped in a flavored coating. The snack is often associated with the cuisine of the American South where they are traditionally accompanied by an RC Cola. Today, MoonPies are made by the Chattanooga Bakery in Chattanooga, TN.
     
Google Answers
Subject: Re: For mathtalk-ga ONLY
From: ddelphi-ga on 13 Feb 2003 08:37 PST
(...)
Q: What do you get when you take green cheese and divide its circumference by its diameter?
A: Moon pi.
 
15 March 2007, McClatchy- Tribune Business News, “St. Paul Academy And Summit School / THEIR LOVE FOR PI IS IRRATIONAL, BUT REAL: At one St. Paul school, the never-ending math constant is a source of infinite fascination, so students and teachers aren’t shy about celebrating 3.141592653589?” by Doug Belden, pg. 1:
A pretty good pi joke: What do you get when you take a circle of green cheese and divide its circumference by its diameter? Moon pi.
 
Twitter
Helenatrandom‏
@Helenatrandom
Q: What do you get when you take green cheese and divide its circumference by its diameter?
A: Moon Pi. #pi
8:06 PM - 23 Jul 2009
 
Twitter
Pat Sawyer‏
@healthyincome4u
What do you get when you take green cheese and divide its circumference by its diameter? Moon pi. LOL!  http://bit.ly/8ImvEZ
2:45 PM - 7 Jan 2010
 
Google Groups: alt.astronomy
Pi Day
Richard A.
3/14/10
* Q: What do you get when you cut a jack o’lantern by its diameter?
* A: Pumpkin Pi!
 
* Q: What do you get when you take green cheese and divide its circumference by its diameter?
* A: Moon Pi.
 
Facebook
MoonPie
April 29, 2012 ·
Another MoonPie joke: What do you get when you take green cheese and divide its circumference by its diameter?
Moon pi.
   
My Town Tutors
Pi Day Jokes: 101 Pi Day Jokes for Math Teachers
Posted on March 9, 2014
(...)
14. What do you get when you take green cheese and divide its circumference by its diameter?… Moon Pi.