SPEAKER 1: This is the iconic CS50 phone book tear. David J. Malan does it every year, but what does it actually mean? 

He does it to illustrate an efficient algorithm, a concept that's very important to computer science. Let's say you need to find a name in a phone book, an inefficient way to do this would be to check every page until you find the right one. That would take ages. 

An efficient way, however, would be going to the middle of the phone book and seeing if the name you're looking for is contained in one or the other half. Once you know which half you need, you can literally throw away the other half. 

You do that until you get to the name you need and ultimately end up saving tons of time, or in the case of CS, computing power. Check out CS50 Lecture 3 on YouTube to learn more about algorithms.