Pi can be used to calculate many different things that are round, for example, pizza. If you wanted to make sure that every person had exactly the same amount of pizza,
you would just have to use a protractor to measure out at which degree you should cut the slice of pizza. That's the traditional way. However, what if you wanted to cut it
into a slightly fancier slice, rather than the boring triangular shape? For example, what if you wanted to make it so everybody had circular pieces?
What you would have to do is measure out the pizza, and determine how many slices of pizza you wanted. For example if you wanted 3 slices,
you would measure the area of the pizza, then find the area such that the area of the outer ring is equal to the area of the inner ring is equal to the area of the small circle
in the pizza. This ends up being such that the radius of the inner ring is (√ 3)/3 r where r is the radius of the whole pizza. The small circle in the pizza is (√ 6)/3 r where again r is in respect to the radius of the entire pizza.
The mathematics involved in finding that is found on pages 199-200 of Pi: A Biography of the World's Most Mysterious Number.
The pages following show two more ways to cut a pizza into fancier cuts while letting the areas of the slices stay the same area.