Pi is a mathematical constant which is the ratio of the circumference of a circle to its diameter (and something about a square and a hypochondriac if I remember correctly!). If that's too hard to swallow, just think of it as a never-ending number which is generally taken to its third decimal place for most mathematical computations: 3.142.
Note that I said 'generally'! Another general, the General Assembly of Indiana, decided that dividing a number by all those decimal points (even three were too much for them, apparently!) was too much for anyone to have to deal with and passed a Bill in 1897 that the value of Pi would now, and forevermore, be the nice round number '4'!
Now... I know what you're thinking! The last time you used Pi was never! But a lot of people use it every day! And for the State of Indiana, every mathematical and engineering calculation was disastrously wrong!
To put this into an everyday perspective, rounding Pi to '4' would mean that a pendulum clock would gain 15 minutes every hour. Just imagine what that would do to the Space Program!
