Q. What is Pi Day and why do kids celebrate?

A. Take anything round -- the wheel of a car, the trunk of a tree, a pillar or a column -- and take a string to measure the distance around the outside, or the circumference. Then, measure the distance across the circle, or the diameter. "If you divide the length around by the length across, you always get the same number," says Glen Whitney of St. James, who is president of the National Museum of Mathematics in Manhattan. "It doesn't depend on the size of the circle. It's somehow an intrinsic property of circle-ness."

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That number is about 3.14 -- which is why Pi Day is celebrated on March 14 every year, 3/14. In fact, however, Pi is an irrational number that continues out myriad digits. So this year, 2015, Pi Day is especially special: at 9:26:53 a.m. and p.m. on March 14, the date and time will represent the first 10 digits of Pi: 3.141592653. The Museum of Math has dubbed it the Pi Day of the Century. Since Pi Day falls on a Saturday this year and kids don't have school, parents can take them to the museum for its daylong celebration. Visit nwsdy.li/pi for details.

"It's a day to stop and appreciate the wonder and joy and mystery that is the world of mathematics," Whitney says of Pi Day. Pi is a Greek letter that has come to be used as the mathematical symbol to represent the number.