Mobile game Threes is addictive and perfect for short bits of gameplay. Its developers introduced an ad-supported free version on Thursday. Screenshot

"Threes" is an addictive little puzzler for iOS and Android, and it's had quite a bit of success for an indie game, notching 1 million downloads. Apple even named "Threes" its iPhone game of the year in 2014. But its developer, Servo LLC, thinks the mobile game could have an even larger audience than it currently enjoys, and is now introducing a free version to go alongside the current $2.99 app.

The game asks you to slam numbered tiles into each other on a grid -- when two tiles with the same number meet, they combine into one tile, creating a bigger number. Bigger numbers mean more points, and getting a high score is the closest thing to winning "Threes." You can download "Threes! Free" for iOS and Android now.

The story of "Threes" turns out to be a story about app store economics. "Threes" discovered a kind of engaging gameplay that was perfect for mobile devices, but it had trouble with copycats. The game "2048" came out shortly afterwards with the same basic gameplay mechanic as "Threes." While "2048" arguably watered it down, ultimately there were a lot of people who downloaded "2048" instead of "Threes" simply because it was free. And "2048" was one of several "Threes" clones available in app stores.

"2048 has mostly served as a constant, overwhelming reminder of our mistake," Threes designer Asher Vollmer told the Verge.

"Threes! Free" doesn't use in-app purchases. Instead, if you want to play a round, you can watch an ad first. Watching a 30 second spot earns you three plays. Even better, the ads don't interfere with gameplay: The only time you see an ad is when you request one to get more plays. Think of it like an arcade, but instead of plunking down quarters, the game wants your attention. And if you want to play ad-free, you can still download "Threes" for $2.99.

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Watching a 30 second ad earns three more rounds of Threes under its new free version. Screenshot