This was CS50x Puzzle Day 2025

By Yuliia Zhukovets, Doug Lloyd, David Malan

15 min readApr 10, 2025

That’s a wrap on CS50x Puzzle Day 2025! This year, over 12,000 participants from 166 countries joined us for what one past participant perfectly described as “a great way to scratch your brain”! We’re thrilled by the turnout and can’t wait to celebrate some of the highlights—stories, stats, snapshots, and more—from this global adventure.

This year was a special one — we celebrated our tenth CS50x Puzzle Day! The event first started in 2016 as an extension of our on-campus Puzzle Day at Harvard (and Yale!) and, since then, it has become a worldwide tradition. Despite the name, Puzzle “Day” actually lasts all weekend — and a bit more, allowing plenty of time for friends, family, and colleagues to take on the “wonderful event that turns your brain inside out!”. We indeed spread it over a weekend that starts when the first time zone on Earth enters Friday and when the last time zone on Earth exits Monday. Just a few days ago, at precisely 00:00:00 on Friday, April 4, this year’s registrants received their puzzle packet— crafted by CS50’s brilliant alumni and friends at Meta: Laura Trutoiu, Zach Zagorski, Cameron Bates, Amy Qiu, Ryan Liu, Helen Chen, Kai Yuan Chi, and Jacqui Fashimpaur.

Results

This year, an impressive number of 12,707 participants registered for CS50x Puzzle Day 2025! We were especially excited to welcome 9,346 first-time participants, another 1,231 people returned for their second round, 451 participants came back for a third, and 587 puzzlers joined us for the fourth time or more. And a huge shoutout to a truly legendary group of 36 individuals who shared that they’ve taken part in every single CS50x Puzzle Day to date!

Among the registrants were

2,070 registrants from Pakistan
1,864 registrants from the United States
1,855 registrants from India
1,202 registrants from Iran
419 registrants from the United Kingdom
330 registrants from Canada
277 registrants from Brazil
200 registrants from Germany
191 registrants from Bangladesh
172 registrants from Ukraine

and so many others from other countries around the globe!

Here’s a quick glance of how all the team scores this year. Each team that provided correct answers to a majority (at least five) of the nine puzzles will be awarded a CS50x Puzzle Day 2025 certificate — if you’re among these teams, keep an eye out for an email from the CS50 Bot in the coming days! (Email puzzles@cs50.harvard.edu if it doesn’t seem to arrive, but do check your spam folder first!)

Our submission form tracked submissions from 2,269 teams and individuals (give or take!), and even more may have participated without ultimately submitting the solution form. Here’s a breakdown of how many teams scored correct answers on each puzzle:

Overall, 1,250 teams correctly found the solutions to all puzzles, including the meta puzzle itself. 1,980 teams correctly solved at least one puzzle!

We also invited teams to share their thoughts on the puzzles — and as you’ll see below, Bomb Squad emerged as the crowd favorite.

When it came to the easiest puzzle, though, one stood out above the rest — Around the World was the runaway winner!

And similarly clear winner for the hardest puzzle was Ticket to Ride.

Solutions

And here are the solutions themselves! Note that we accepted multiple solutions for some of the puzzles.

Ticket to Ride accepted TRANSACT.
Bomb Squad accepted OFFWHITE.
Tangrams accepted CHILIDOG.
Wavelength accepted INFRARED.
The Blind Maze accepted CEREBRAL.
Powers of Two accepted DISCOVER.
Around the World accepted ISTANBUL.
Banananagrams accepted EGGPLANT.
Codenames accepted LOSTINPAIROFDICE and PARADISE and LOSTINPARADISE.

Not sure how to get to those answers? Check out the step-by-step walkthrough of puzzles below:

Playlist of solutions

Memes

Now a CS50x Puzzle Day tradition, memes expressed both frustration and joy.

Participants’ Reflections

Considering joining us next year? Here’s what a few participants had to say:

“A perfect blend of logic, creativity, childishness, and a dash of crazy.”
“Chomp! Chomp! Chomp! Your brain needs to eat… so feed it!”
“Unbelievably difficult yet extraordinarily satisfying.”
“Do it!! The mental gymnastics and gyrations are absolutely worth it!!”
“Come for the puzzles, stay for the friends and fun.”
“Enjoy the games, and if you get stuck, try to find another way to solve the puzzles. Sometimes, the answer is easier than it seems.”
“Had a great time teaming up with awesome people from different places — solving puzzles, laughing at dead-ends, and celebrating every breakthrough!”

Now to conclude, thank you to everyone around the world who took photos (and screenshots) of their teams solving puzzles!