Google is celebrating the 40th anniversary of the Rubik's Cube by changing the logo on its homage into a playable version of the puzzle. On reaching Google's home page, users will see the red, blue, yellow and green Doodle of Google's logo on the Rubik's Cube. Users can also click on the Rubik's Cube, which then spins and groans into a large interactive, online version of the game. Players can click to rotate and change the different sides of the cube, and Google will count the number of moves users take to complete the puzzle. Monday is the 40th anniversary of the popular puzzle, which was created by Hungarian architecture professor Erno Rubik. Initially called the Magic Cube, it was renamed after its inventor and went on to become the most popular kids toy in the 1980s. An estimated 350 million Rubik's Cubes have been sold worldwide. "It was wonderful to see how, after only a few turns, the colors became mixed, apparently in random fashion," Rubik, a 29-year-old at the time, once said. "It was tremendously satisfying to watch this color parade."