Working away here, I almost didn’t notice the date: March 14, or 3/14. It’s Pi Day, the offbeat holiday in celebration of our favorite mathematical constant, pi—a.k.a 3.14159, etc. Swiss mathematician Leonhard Euler adopted the symbol, π, in 1737, but the concept has been in use since ancient Greece; it’s also known as Archimedes’ constant. Popular Pi Day activities include eating pie and listening to Kate Bush’s song “Pi,” in which the singer musically recites 137 digits of the constant, leaving out the 79th through 100th decimal places along the way. The song is almost as difficult to listen to as pi is to derive. Here’s a celebratory trivia question: Why was the Greek letter π chosen to represent this important mathematical constant, and who proposed it? I’ll send an issue of Popular Science autographed by the soup vendor outside our office to the first person to answer correctly. (Enter your responses in the “Comments” section below, please.) —Joe Brown