"A slice of pi" Key Ring

Pi relates the circumference of a circle to its diameter. Its just the ratio of the circumference and diameter (pi = C/d). This means that the circumference of a circle is always equal to its diameter multiplied by pi. This works for any circle of any size, which is pretty incredible when you think about it! Even more incredible when you consider that pi is an irrational number, meaning it cannot be represented as a ratio of integers, so the diameter can never exactly divide the circumference, instead resulting in an in infinitely non-repeating decimal. This universal property of circles means that pi finds its way into many areas of mathematics and therefore physics, engineering and many other disciplines. Essentially you will encounter pi anywhere you find circles, or spheres, or rotations, or steady fluctuations in time (known as harmonic motion) and a whole array of related phenomenon.