Harry knew pi to 3.141592 because accuracy to one part in a million was enough for most practical purposes. Hermione knew one hundred digits of pi because that was how many digits had been printed in the back of her maths textbook.
9. Memorize the first 3 digits of pi, then make up the rest. No one will care enough to check if you are right.
"Pi. The ratio of the circumference of a circle to its diameter. And this is just the beginning. It keeps on going—forever, without ever repeating. Which means that contained within this string of decimals is every single other number. Your birth date, combination to your locker, your Social Security number. It's all in there somewhere. and if you convert these decimals into letters, you would have every word that ever existed in every possible combination. The first syllable you spoke as a baby, the name of your latest crush. Your entire life story from beginning to end. Everything we ever say or do... all of the world's infinite possibilities rest within this one simple circle. Now, what you do with that information... what it's good for... well, that would be up to you."
— Harold Finch, Person of Interest, "2-Pi-R"
