Happy Pi Day -- a day people all over the world commemorate the mathematical constant also known as 3.1415926535 . . . (and so on).

From Michael Caine to Billy Crystal, Chris Klein to Taylor Hanson, some famous faces are celebrating Pi birthdays today.

Pi Day, which was recognized by the US House of Representatives only two years ago, was created by San Francisco physicist Larry Shaw in 1988.

Now, 24 years later, Shaw's Pi Day is still going strong, with a line-up of activities that go beyond just eating a slice of pie (not that there's anything wrong with pie).

According to the San Francisco Exploratorium website, Pi Day this year will begin just before 1 p.m. (PST) with a special homage to Albert Einstein, who would have turned 133 years old today. Shaw will then discuss the history of Pi, complete with Pi limericks and a demo.

Meanwhile on Twitter, Happy Pi Day has been a trending topic since early Wednesday morning. From fun facts about the famous mathematical constant to musings about eating pie, folks are celebrating everywhere.

Here are a few famous faces celebrating Pi birthdays today. (Let's hope they'll have pie and cake!)

Billy Crystal
Nine time Oscar host (the most recent gig being last month’s Academy Awards) turns 64 years young on Pi Day. REUTERS
Michael Caine
Sir Michael Caine turns 79 years old today. The legendary actor is getting ready for a big year, with the third and final installment of Christopher Nolan’s Batman film series, “The Dark Knight Rises,” out July 20. REUTERS
Quincy Jones
Also turning the big 7-9 is record producer/music extraordinaire Quincy Jones. REUTERS
Chris Klein
It’s hard to believe that “American Pie” came out 13 years ago. Now, Chris Klein and the gang are all back for “American Reunion,” due in theaters Apr. 6. Klein, who plays the lovable tough guy Oz, turns 33 today. REUTERS
Taylor Hanson middle
Speaking of time flying by, little Taylor Hanson of Hanson is not so little anymore. The Tulsa father of four turns 29 years old on Pi Day. REUTERS
Jann Browne
Country singer Jann Browne turns 58 today. Since her big ‘90s hit, “Tell Me Why,” the Indiana native has been busy managing her own record company, plan B records, which produces west coast country, Americana and roots music http://www.jannbrowne.com/
Kristian Bush
Happy 42nd birthday to Kristian Bush of Sugarland! The band is getting ready for a tour later this spring in support of Sugarland’s latest, “The Incredible Machine.” REUTERS
Albert Einstein
Happy 133rd birthday to the father of modern physics -- Albert Einstein. REUTERS