From the Super Mario Wiki
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Face Blocks are blocks that appear in Super Mario Bros. Deluxe's Vs. Game and You vs. Boo modes. At the start of the stage, they appear as White Face Blocks, which are smiling, and in later stages, cause certain Dotted Line Blocks to become solid white blocks that help the player, before changing into Red Face Blocks, which have their tongue sticking out and cause most Dotted Line Blocks to become solid red blocks that generally block the way. Jumping into a Red Face Block causes it to turn into a white one, turning all of the solid blocks back into Dotted Line Blocks for a few seconds before both automatically revert back; the player can also hit a White Face Block to instantly turn it back into a red one.