SAN FRANCISCO--If ever a mathematician could be excited about a date, today would be it.

That's because it's Pi Day, March 14, or, for you Americans, 3/14. And since pi--the ratio of a circle's circumference to its diameter--is casually said to be 3.14159, today's the big day.

That's why I spent a big part of my day at the Exploratorium here for the science museum's 20th annual Pi Day celebration.

The brainchild of longtime Exploratorium employee Larry Shaw, the day's festivities celebrated the popular irrational number with a series of pi-related activities and events, mainly aimed at kids, and even one very small special guest.

Among the items on the agenda Friday was an "ask a scientist's Pi Day puzzle" session, a Pi procession, a "why Pi" lecture, and a demonstration by a world-champion pizza tosser.

So I thought I'd start my day at the Exploratorium by asking Shaw, a longtime Exploratorium employee and the originator of Pi Day in 1988, why pi means so much to so many people.

"It's how to get from one dimension to another," said Shaw, referring to the fact that the formula for determining pi involves both a two-dimensional object, a circle's circumference, and one with only one dimension, the circle's diameter.

He added that pi means a lot to so many because of its metaphysical properties. It's irrational, he said, and transcendental, and while there's a procedure for how to calculate it, it's also infinite.

"You can find the next number," Shaw added, "but you can't predict it."

Later, I asked Paul Doherty, the Exploratorium's senior scientist, why *he* thought pi is so exciting to so many people.

"There are people in the world who are interested in rock and roll and who are interested in sports," Doherty said. "We're just interested in mathematics, and we want to provide really good, interesting information for those people who have a deep interest in mathematics...and for those who don't know they might."

Doherty said pi is "this amazingly simple-to-define thing that produces a number that's just never ending, and this idea that something so simple can generate a number that's never ending is just amazing."

In part, pi's popularity has to do with its randomness, Doherty said. He pointed to other infinite numbers, like one-third or the square root of 2. But he argued those numbers are boring.

"With pi," he said, "it not only never ends, but it never repeats."

Of course, in some places, pi actually does repeat. Doherty pointed out that somewhere, hidden in the first 2 billion digits of pi, there's eight eights in a row, as well as a sequence of 1, 2, 3, 4, 5, 6, 7, 8, and 9.

But, he pointed out, those are just pure coincidences, a function of the probability of getting any kind of sequence of numbers in a string currently over a trillion digits long.

And speaking of strings, the term means different things to Shaw. Among the attractions he offered Friday was the "Pi string," a long string of colored beads that represents the first 4,000-plus digits of pi.

Over the years of doing Pi Days, Shaw has had participants add more beads to the string, and on Friday, he had set up 10 cups with beads for people to add to the string based on a print-out of thousands and thousands of digits. Each cup contained a single color bead and each color represented a single digit.

One year, he said, someone put one of the wrong colored beads on, but he was able to fix the string before anyone else noticed.

As to how anyone would have the attention to detail to notice something like that, well, that's another matter. But for people like Shaw--and mathematicians in general--it's precisely this detail that matters when dealing with something as both exact and inexact as pi.

One Pi Day attendee who may not yet appreciate the significance of the number is Pi Barker, the son of Chad and Robyn Barker.

The two named their new son--he was born December 6, 2007--Pi because of what they saw as the metaphysical meanings associated with the number.

"Since pi is an infinite decimal," said Robyn Barker, "it's a gift for us to give him. So wherever he is, it's a reminder of his infinite possibilities."

Pi's father, Chad, who is a manager in Google's strategic partnerships group, added that he and his wife appreciated giving their son a name that calls to mind the circular nature of life as well as the never-ending nature of life.

As to whether their families and friends appreciate the name Pi, that may still be to be determined. Especially when little Pi makes it to school and has to contend with the inevitable cruel nature of little children.