From the Super Mario Wiki
A Block is a type of block in the Mario series. In Super Mario Bros., it is said that Toads have been turned into ? Blocks, Brick Blocks and other blocks. Blocks (the "used" ones) usually appear when Mario or Luigi hit a ? Block, Brick Block with an item or coin, or a Coin Block, though they sometimes are naturally Blocks. Blocks are usually square and brown with rounded edges and nails. In Super Mario World, Blocks have small, closed, angry eyes, and if the player hits a blue P-Switch, all Blocks turn into coins and vice versa for a short time. Blocks can only be broken by Mega Mario or stronger enemies, such as Broozers.
Giant Blocks appear in Super Mario Bros. 3, Super Mario Galaxy 2, and New Super Mario Bros. U. In Super Mario 3D Land, Giant Blocks are usually found in hidden rooms accessed via Warp Pipes, and are often put together to form sculptures that create 3D illusions.
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