What goes better with mornings than coffee? And what goes better with coffee than pie? Today, of all days, is the perfect day to enjoy a slice of pie with your morning coffee—it’s Pi Day!

Apple pie and coffee for breakfast. Image Credit: Lizi Beth (CC BY 2.0) 
Celebrated on March 14th (3/14) of every year, Pi Day is a nod to the mathematical constant π. Usually rounded to 3.14, π appears in many of the equations that describe our world. You probably know it best in the context of circles: the area (A) of a circle with a radius of r is given by A=πr^{2}. The circumference (C) of a circle is given by C=2πr. In fact, pi is defined as ratio of the circumference of a circle to its diameter (d), π=C/d. Some people say that 2π, a constant called τ (tau) is more useful and elegant than π, but the philosophy is slow to catch on—probably because “tau day” (held on June 28th) doesn’t double as an excuse to eat flaky, delicious pastries.
Image from Wikimedia Commons (CC SA 3.0) 
Circles aside, π is a particularly intriguing number because it comes up in many different mathematical situations. In physics, π plays a role in determining the period of a simple pendulum, the force between two electric charges, Einstein’s cosmological constant, and Heisenberg’s uncertainty principle, to name a few examples. (Interestingly enough, March 14th is also Einstein’s birthday!)
Not only is π common, it’s irrational. Irrational numbers can’t be written as fractions. That might not sound very exciting, but it means that π goes on and on forever—there is no end to the sequence of numbers that come after the decimal point, and there’s no discernible pattern to them, a fact which has some interesting implications. Pi has been calculated to more than one trillion numbers after the decimal point, just for fun. There’s really no practical reason that we need to know the number that precisely.
For the perfect Pi Day, food is important. I recommend pie for breakfast, but don’t stop there. Pizza pie, chicken pot pie, and shepherd’s pie are great meal options. Many places have specials on these items today if you don’t feel like cooking. And don’t forget dessert! Several stores and restaurants have deals on pies in celebration of Pi Day—my local grocery store has them on sale for $3.14 off the regular retail price.
Aside from eating, make sure that your day includes at least a bit of fun math. Here are a few options that range from actual calculating to an artistic interpretation of π:
• Before you cut into your pie, measure its circumference and diameter, divide, and see how close you get to π.
• Take the “Pi in the Sky Challenge” by NASA’s Jet Propulsion Laboratory and see if you can solve some stellar math problems using π.
• Prefer brain teasers? Check out The Guardian’s Pi Day puzzles and see how you do.
• Estimate pi by throwing frozen hotdogs. For real. (If you try it, let us know how it goes. Photos appreciated.) For a less messy version, you can do the same thing with matchsticks or toothpicks!
• Get a sense of the many decimals places of π and decorate your house, office, or classroom at the same time with a colorful Pi Day Paper Chain. This chain includes one link per decimal place and one color per digit.
After a long March 14th of eating and mathing, unwind with some catchy pi tunes. Or if you’d prefer some charttoping hits, you can to listen to some pie tunes instead. Sure, the songs may be gimmicky, but π is a very real number that comes up again and again in our quest to understand the world. Taking a few minutes each year to consider what this means and reconnect with the math is a worthwhile exercise. It’s also a good excuse to eat some pie.
—Kendra Redmond