# National Pi Day

**What Is Pi?**

Pi is the circumference of a circle (the distance around the circle) divided by its diameter (the distance across). In other words, the circumference of any circle is approximately 3.14 times its diameter. Because pi is an irrational number, it has an infinite number of digits. No matter how many decimal places we calculate, pi will always be an approximation.

Because pi is the same for every circle, we can use it to determine the diameter if we know the circumference, or vice versa. And when we know the diameter, it’s easy to calculate the area.

This__ gif (animated sequence) that "unrolls" pi __will give your students a quick visual of how a circle’s diameter, circumference, and pi are related.

**The History of Pi**

From ancient China to Babylonia, mathematicians have been puzzling over pi for thousands of years. Since pi goes on forever, even the most powerful supercomputers will never know all of pi’s digits. But that doesn’t stop people from trying! In 2002, a team of mathematicians at the University of Tokyo broke the record, using computers to calculate pi to 1,241,100,000,000 decimal places. Check out this amazing display of the first million digits of pi.

## Teaching Pi with Pie

Every year on March 14, math lovers around the country celebrate pi. Some enthusiasts even go so far as to mark Pi Minute at 1:59, which takes into account the first six digits of the pi: 3.14159. Pi Day provides the perfect excuse for incorporating math and geometry into all aspects of the school day.

If you are not sure how to explain “pi” to your class, or if explaining it is met with blank stares, we know just how to do the job! Pie! Watch the video below to get a yummy tutorial on why circumference divided by diameter equals delicious.

https://www.youtube.com/watch?time_continue=77&v=RzfltUSvmWM

Obviously, the best way to teach pi… is to bring in pie! Here is a scrumptious recipe for apple pie:

## Ingredients

### Pie Crust:

2 1/2 cups all-purpose flour

1 cup of unsalted butter. Chilled, cut into tablespoon size pieces.

½ teaspoon salt

7 tablespoons of ice water

1 tablespoon cider vinegar

## Apple Pie Filling

### 9-inch round pie pan

1/2 cup unsalted butter

3 tablespoons all-purpose flour

1/4 cup water

1/2 cup white sugar

1/2 cup packed brown sugar

8 Granny Smith apples - peeled, cored and sliced

## PIE CRUST

1. Combine flour, salt, and butter in a food processor. Pulse until mixture resembles coarse crumbs, about 10 1-second pulses.

2. Stir water and vinegar in a small bowl.

3. Pour half the ice water and vinegar mixture into the flour and butter mixture.

4. Pulse to combine, about 3 (1-second) pulses. Pour in remaining ice water and vinegar mixture.

5. Pulse until mixture just starts to come together, about 8 (1-second) pulses.

6. Turn dough out onto a wooden surface, pat into round shape and divide in half.

7. Form each half into a disc about 5 inches wide.

8. Wrap each disc in plastic wrap and refrigerate for at least 30 minutes until ready to use.

## PIE FILLING

9. Preheat oven to 425 degrees F (220 degrees C).

10. Melt the butter in a saucepan. Stir in flour to form a paste.

11. Add water, white sugar and brown sugar, and bring to a boil.

12. Reduce temperature and let simmer.

13. Place the bottom crust in your pan.

14. Fill with apples, mounded slightly. Cover with a lattice work crust.

15. Gently pour the sugar and butter liquid over the crust. Pour slowly so that it does not run off.

16. Bake 15 minutes in the preheated oven. Reduce the temperature to 350 degrees F (175 degrees C). Continue baking for 35 to 45 minutes, until apples are soft.