From the Super Mario Wiki
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Yellow Blocks are objects seen in the Paper Mario series; they can be destroyed with the basic hammer. In Paper Mario and Paper Mario: The Thousand-Year Door, they are not very hard to get by, with Stone Blocks and Metal Blocks presenting more difficult obstacles. In the latter game, however, a new type of Yellow Block is also present, being much larger, having angry eyebrows, and requiring a swing from the Super Hammer in order to be destroyed. In the original Paper Mario, Kammy Koopa is shown summoning one of these in to interfere with Mario's progress.