SAN FRANCISCO (KGO) -- Pi symbolizes the ratio of a circle's circumference to its diameter, or 3.14, as in today's date.
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Pi Day started at San Francisco's Exploratorium and it's the 30th anniversary. To celebrate, there will be tons of free events starting at 10 a.m.
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Click here for a list of event for Pi Day at the Exploratorium.
A child looks over an exhibit at the Exploratorium in San Francisco, Tuesday, April 9, 2013. (KGO-TV)