From the Super Mario Wiki
Blockhoppers are robotic enemies encountered in the game New Super Mario Bros.. They are found only on World 2. They disguise themselves as a ? Block, with Brick Blocks on top of them, but once Mario approaches, they reveal their true form. They are actually blue robot-like enemies that hops at Mario in cue with the background music. They can be told apart from ? Blocks by the fact that the question mark on them does not move. They are similar to the Pile Driver Micro-Goombas that disguise themselves as blocks from Super Mario Bros. 3. They can be defeated with either a thrown Koopa's shell, a Ground Pound, or by running into them using the Shell Dash ability using the Blue Shell. Mario gets a coin or an item if he Ground Pounds the ? Block on the Blockhopper.
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