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 Question Blocks, Brick Blocks and other blocks. Blocks (the "used" ones) usually appear when Mario or Luigi hit a Question Block or Brick Block with an item or coin(s) inside, 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.