You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400π m. what is the diameter d of the lake? d=

Here. If you take a piece of string that is 2 units in length (lets say meters) and make it into a circle (as close to a perfect circle as you can), then the radius of the circle will be 1/π because circumference is 2 * π * r. Basically, anything involving circles will have pi in there somewhere, because pi is a ratio, it is simply the circumference of a circle over the diameter of the circle. This works for any size circle.

Other things that involve pi are sin, cos, and tangent functions, which are used often in physics, where you are investigating something that has a cyclic nature or some kind of circular motion. (like a wheel turning, a sound wave, etc)