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.

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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!)