Quick, what's the No. 1 downloaded free app on iTunes this week? If you answered QuizUp, you're one up on the competition.
The new title from Reykjavik, Iceland-based Plain Vanilla Games has rocketed up the iTunes free app chart to claim the top spot with more than 1 million people downloading the game during its first week of release.
QuizUp is, no surprise, a quiz game, where players compete in real-time, head-to-head trivia battles.
This is how it works: You sign on, pick one of more than 300 topics, and either challenge somebody you know or let the game choose an equally ranked opponent for you. The two players are shown 10 questions in a row and have only seconds to pick each right answer.
Sounds simple, right? So why is QuizUp so much more popular than other quiz games?
Thor Fridriksson, co-founder and CEO of Plain Vanilla Games, claims it's the game's social aspects, like the ability to send notes to your opponent in-between rounds.
"Trivia is one of the most inherently social activities you can do," Fridriksson, 34, told NBC News. "When families are watching 'Jeopardy' on TV and somebody knows an answer, they shout it out. People want to show that they know stuff."
That would explain why you can't play QuizUp on your own. Fridriksson described the game as almost a social network, with people chatting with each other after matches, challenging their Facebook friends and leaving at least a hundred thousand posts on the game's discussion boards over the last two days alone.
Another reason the game has been so successful, according to Fridriksson, is the plethora of questions, currently at 160,000 and climbing.
The 20-person Plain Vanilla Games team doesn't sit around all day thinking of trivia. Instead, QuizUp acts like a "gamified Wikipedia," asking people to submit 10 sample questions in their particular area of expertise. So far, Fridriksson said, more than 1,000 people have applied to be contributors, providing a stream of fresh content that his small staff could never produce.
"The reason why Trivial Pursuit only had six categories is because they printed their content on cardboard," he said. "People don't get that passionate about broad topics like entertainment; they get excited by specific singers like Justin Bieber."
(That tween appeal doesn't stop with the Biebs. One of the company's first corporate partnerships was a quiz game tied to the release of "The Twilight Saga: Breaking Dawn - Part 2.")
Despite the fact that QuizUp was only released on Nov. 7, more than 10 million games are now played every day, the company claims, with the average person on the app for 40 minutes per session.
Plain Vanilla Games started with Fridriksson and two programmers going on a self-described three-month "Viking journey" from Iceland to San Francisco in 2012, where the company scored $1.2 million in seed funding.
Now they are planning to unveil an Android version sometime around Christmas and hope to release new features like large, friends-only tournaments and QuizUp topics based on standardized testing questions, so people preparing for the SAT or GMAT can have a little fun while studying.
In the meantime, Fridriksson said, the tiny team is just trying to keep the game's current 1.5 million users happy.
Keith Wagstaff writes about technology for NBC News. He previously covered the tech beat for TIME's Techland and wrote about politics as a staff writer at TheWeek.com. You can follow him on Twitter at @kwagstaff and reach him by email at Keith.Wagstaff@nbcuni.com.