Visitors to the CS 50 Fair on Dec. 9 were welcomed — or, rather, pelted — with baby-blue stress balls. The greeting, by laughing students in Angry Birds costumes, gave way to a bustling scene — crowds, music, candy, popcorn, and (of course) hundreds of laptops.The event at the Northwest Science Building represented the culmination of the introductory computer science course known as “CS 50.”Consistently one of the most popular undergraduate courses at Harvard, CS 50 defies convention. A large portion of the class had stayed up all night the previous weekend at the Microsoft-sponsored Hackathon. To resolve any last-minute bugs before their projects went live, students helped one another, aided by pizza at 8 p.m., Chinese at 1 a.m., and pancakes at 5 a.m. (for anyone still typing).“It’s always striking, just how much they end up teaching themselves by semester’s end,” said instructor David Malan, senior lecturer in computer science at the Harvard School of Engineering and Applied Sciences (SEAS). “Most of the projects here end up diving into some territory that we simply don’t cover in the class.”CS 50 attracts students from all class years and backgrounds: 76 percent of those enrolled had never taken a computer science course, and 10 percent were students at Harvard Extension School.Projects ranged from a tool that limits procrastination, to a website that displays longitudinal market capitalization data, to an application that helps with music composition.“With the final project we finally take the training wheels off and ask students to just go out and create something — anything — using whatever tools they like and using the skills that they’ve obtained over the course of the semester,” said Malan (A.B. ’99, S.M. ’04, Ph.D. ’07), who took CS 50 as an undergraduate at Harvard College. “It’s very rewarding. I remember it as a student, and it’s easy to see as a teacher now, looking at students, how excited they are when they get something to work. It’s amazing how gratifying it is when they finally go out and create something of their own.”The class has given rise to many websites popular among students, including HarvardFML (a social complaint board) and ISawYouHarvard (a site for missed connections), as well as practical applications such as Shuttleboy (an app to help track the arrival of the next campus bus) and a course-shopping tool.Each new crop of students is encouraged to create projects that will benefit the campus community.Nuseir Yassin ’14 drew inspiration for his project, Kindify, from the course itself. An economics concentrator, Yassin was fascinated by the way one act of kindness — for example, helping a classmate with a problem set — could set off a chain of helpful behavior throughout the class. Yassin created a “tree of kindness” that allows people to watch their kind acts spread, providing game-like incentives to connect with others in a network of goodwill.Christine Chen ’14, an economics concentrator, and her partner, Zijian Wu ’14, (molecular and cellular biology) worked to create change slightly farther afield. For their final project, they created a website for the Chinatown Citizenship Program, of which they are co-directors. The organization, affiliated with Harvard’s Phillips Brooks House Association, holds tutoring sessions in English, history, and civics for Chinese immigrants who are studying for the U.S. citizenship test.“We wanted to use this as a publicity tool,” said Chen. “Our prospective students and tutors can find out more about us and get involved with us.”Chen, who had very little experience in computer science before enrolling in CS 50, decided to take the leap because so many of her upperclassmen friends had recommended the course.“I was a little scared, but David Malan did a really good job letting us know that a lot of us are inexperienced, so we feel like we’re not alone,” she said. “There are some really good resources, so we can manage it even without prior experience.”Head teaching fellows Matt Chartier ’12 and Rob Bowden ’13, armed with iPads, coordinated a small army of 98 teaching fellows and course assistants to manage office hours and homework assignments.“I’ve really gotten to see the fair develop from its inception to what we see here today,” said Chartier, who took the class as a freshman. “It’s a way for all the students to share what they’ve worked on throughout the course with their friends from across the campus.”More than anything, he said, the CS 50 Fair is a chance “for people to be able to come together and see what CS is all about — what computers empower people to create.”Matthew Chartier ’12, a computer science concentrator, is one of two head teaching fellows for the course, which enrolled 677 students this term.