From the Super Mario Wiki
Boulders are large rocks found throughout various Mario games, which usually serve as obstacles.
Boulders first appear in Wario Land: Super Mario Land 3, and are found in the SS Tea Cup (one in the first level) and in Parsley Woods (with spikes). They also appear in Super Mario 64 in Hazy Maze Cave. In one room, boulders endlessly roll down a slope and into a bottomless pit. If Mario attempts to climb the slope, the boulders will deliberately try to home in on him, although they can be dodged. The level's sixth Power Star is entitled "Watch for Falling Rocks", and is located near the Boulders. In Super Mario 64 DS, Wario must instead punch several boulders, (or ram them upon growing with a Super Mushroom) to receive the Power Star.
In Donkey Kong 64, Boulders appear as stationary obstacles, often appearing on various pads, which require them to be moved before the pads could be used. However, only Chunky Kong can lift them. Boulders also serve as weights, and are sometimes needed to keep various switches pressed down. Chunky Kong can also throw them at enemies. In this game, they are dark green in color.
In Super Mario Galaxy, they roll around in various patterns, and if Mario touches them, he will be knocked down and lose one wedge of his Health Meter. They are semi-common obstacles that appear in several galaxies, including the Good Egg Galaxy and the Honeyhive Galaxy. They can be destroyed if Mario spins their weak spot: a circular red area. They can also be destroyed if Rainbow Mario collides with them, a Stretch Plant is knocked into them, or if Player 2 uses a second Star Cursor to hold one in place, allowing a second one to collide with it, which will destroy both boulders, leaving behind a large amount of Star Bits.
In Super Mario Galaxy 2, Boulders reappear in the Boulder Bowl Galaxy, a galaxy which is appropriately centered around Boulders, as well as the Rock Mushroom. This time, the boulders can also be destroyed by Rock Mario. The boss of this galaxy, Rollodillo, attacks in the same way as a Boulder, and can only be defeated by using Rock Mario. Sorbetti, the boss of the Freezy Flake Galaxy, also uses a similar strategy during the boss battle.
Boulders make their next sidescroller appearance in New Super Mario Bros. 2. They appear in World 3-5, which is an underwater level. They are common obstacles here but they are also necessary to obtain several Star Coins as they are enclosed in blocks that cannot be broken by Mario.
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