The small flyers cropped up during Camp Yale. They were simple, shaped like a small jigsaw piece, the words “CS50 Puzzle Day” emblazoned across.

CS50, or “Introduction to Computing and Programming,” the joint Harvard-Yale course whose roster boasted 670 enrolled students on its first day of class, is the most popular course on campus. On Saturday, the class brought around 150 Yale students to Harvard for the first joint Puzzle Day competition. The event was classic CS50 — loud music, photo booths, free food and seemingly unlimited swag.

Over 700 Harvard and Yale students competed, solving eight puzzles, which included ciphers and word searches. A longtime tradition of CS50, Puzzle Day, which was sponsored by Facebook this year, is meant to get students excited about the semester ahead. The puzzles require no computer science knowledge, but the environment — nonstop problem-solving — mirrors the cultural symbol of the CS world, the hackathon.

While the competition was friendly, students were quick to make jokes with one another.

“I find it hilarious that the most popular class at Yale is a Harvard class,” said Harvard junior Andrew Mayo, who took fourth place at Puzzle Day.

But one of Mayo’s teammates, Harvard junior Zach Bathon, took a more generous approach. He took CS50 his freshman year, loved it and wants Yalies to be able to experience it, too.

The lecture course, which is being offered in two separate time slots due to high demand, included a class selfie, and has become a kind of a cultural phenomenon. The first day of class included music played by a disc jockey and cake. The Facebook Group “Overheard at Yale” and the Yale Rumpus Twitter page featured many live update posts during the first lectures. One Overheard post even referenced a sarcastic remark made by another computer science professor to his students on the first day of lecture.

“This is not a Broadway show. That’s CS50,” the post reads.

At the Yale Up! pep rally on Saturday, even Yale College Dean Jonathan Holloway compared the boisterous celebration of reveling Yale students to the hype of a CS50 lecture. “I don’t know whether this is Yale Up! or CS50,” Holloway said through cheers.

Students interviewed at Puzzle Day who plan on taking the course acknowledged “extensive hype” surrounding it, but six out of seven students said they believe it is warranted. The special effects and prizes that epitomize the class help students get excited to learn about computer science and encourage those who have never been exposed to computer science before to take the course, said Yingjie Wang ’17.

“I think [CS50] is actually making people excited about learning, and from what I’ve heard from other students at Harvard and who have taken the course online, it is actually a good class,” she said.

Wang, who is still shopping the course, said she wonders whether this initial excitement and momentum will be sustainable throughout the semester. She said she is unsure whether students will be this enthralled midway through the semester, after the cake runs out and the DJs disappear.