A Yellow Block.
Yellow Blocks are objects seen in the Paper Mario series; they can be destroyed with the basic hammer or by using Cudge. 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. However, in Super Paper Mario, Cudge is not obtained until Chapter 5, meaning Yellow Blocks hinder progress for a substantial amount of the game. Yellow Blocks also look similar to Rotating Blocks from Super Mario World.