Pi

Pi is a magic number. It is the constant ratio of the circumference of a circle to its diameter. Pi is irrational i.e. you cannot represent it as a fraction. As a decimal, you would need infinite amount of decimal point precision. It approximates to 3.14 and that is why the 14th day of the 3rd month is Pi Day. So today, have a slice of Pi!
See Also: http://en.wikipedia.org/wiki/Pi_Day

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