Minecraft is deceptively simple video game. You're dropped into a virtual world, and you get to build things. It's like a digital Lego set, but with infinite pieces.

Its simplicity makes it a big hit with kids, like 10-year old Will Davidson. Last year, Will built a Spanish mission for a school report. He modeled his off the Santa Cruz Mission. "I made a chapel over here," Davidson says. "I also have a bell tower."

After he turned in his report, he added a few things. Like skeleton archers. "And zombies ... and exploding things, and spiders, that try to kill you," he said.

Minecraft is popular with kids because they're free to create almost anything, says Ramin Shokrizade, a game designer.

Also, kids aren't manipulated into clicking buttons to buy add-ons within the game. In other games, designers give players a special power for free at first, then take it away and offer it back at a price.

Zynga, the creator of Farmville, calls this fun pain, according to Shokrizade. "That's the idea that, if you make the consumer uncomfortable enough, and then tell them that for money we'll make you less uncomfortable, then [they] will give us money," he says.

Kids, Shokrizade says, are especially susceptible to this — and Minecraft has a loyal following, in part, because it doesn't do it.

Susan Linn, from the Campaign for a Commercial-Free Childhood, agrees. She says a big reason she likes Minecraft is because after you purchase the game upfront, that's it.

"Parents don't have to worry that their kids are going to be targeted for more marketing," Linn says. "How forward-thinking!"

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