# Why Pi Deserves Its Own Day (and How It Can Help You Find the Best Pizza Deal)

## Maybe you met pi in grade school. Maybe you didn't appreciate it. The magical, practical number that goes on forever and forever is celebrated on the day that matches its first digits: 3/14.

Here's a funny story. A reporter at National Geographic wanted to find someone to interview about pi—that number you learned in grade school that is the ratio between a circle's circumference and its diameter.

So the reporter thought, I wonder if there's such a thing as a museum of mathematics.

And the reporter googled "museum mathematics." And you won't believe this, but there really is a Museum of Mathematics in New York City, which opened in 2012.

The museum will, as you might imagine, celebrate pi on what's known as Pi Day. That day is 3/14 because the start of the number pi is 3.14. And since the next three digits are 159, the museum and other museums that love numbers, like San Francisco's Exploratorium, hold special Pi Day events at 1:59 on March 14.

In case you're not a numbers person, this reporter put the tough question to Glen Whitney, founder and director of the Museum of Mathematics: What's so great about pi?

"What pi tells you," he said, "is that there is an underappreciated beauty in math. You can figure out the circumference of a circle just from its radius. No matter how big or how small the circle is, it always has that same relationship. It's a beautiful, simple relationship."

It's also the area of a circle. (The formula for circumference is 2 pi r. For area it's pi r squared.)

The other cool thing about pi, Whitney said, is that "it is not a simple ratio of numbers." It can be expressed as 22/7, but according to Whitney, "that's an approximation. You can never capture it exactly. Its decimal expansion goes on forever."

Like a trillion digits. Humans try to memorize and recite as many as they can. The record: Chao Lu with 67,890.

[Note to self: Ask Chao Lu for tips on remembering combination to my gym locker.]

If you're not a numbers person, you may say, What good is pi to me? Lori Lambertson, an educator at the Exploratorium in San Francisco, proposes a way that anyone can benefit from pi. Say you want to find the best deal on a pizza. Using pi, "find the area of the pizza, then take the price of pizza and divide it by the area to get the cost per square inch." So let's see: 12-inch pizza for \$7.99. Area of the pie: 37.68 square inches. Cost per square inch: approximately 21 cents. Dear Miss Levy, you were my elementary school math teacher. I hope I did it right. Readers, let me know if I screwed up.

Meanwhile, can I just say: Mmmmm, pizza.

If you're very ambitious, the Museum of Mathematics has an experiment you can conduct with chocolate chips and a circle and pi. And then you can bake the chips into a perfect round cookie.

Or perhaps you are a word person. Then the Museum of Mathematics has the perfect pi format for you. This year the museum is holding a "pi-ku" contest, asking people to write poems about pi, with, yes, you guessed it, 3 syllables in line one, 14 in line two, 15 in line three.

Here is a sample entry from a fifth grader:

What is pi?

Can you find out the dilemma? But math's cloak hides it well.

The relationship of equations—the hidden number pi.

Is this kid the next poet laureate or what? The judges will award the prize for best pi-ku today, with the winning composition posted on the museum's website.