From the Super Mario Wiki
Message Blocks, also referred to as Hint Blocks, are blocks with faces that appear in various platformers. They are most prevalent in the Yoshi series. They are usually white, smiling blocks that provide tips and hints to the player.
Super Mario series
Super Mario World
Message Blocks first appeared in Super Mario World, where they usually describe a new level in the beginning stages of the game or explain a new game mechanic. Unlike later appearances, these Message Blocks have no face in Super Mario World but instead look like speakers.
Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Message Blocks became more common in the Yoshi series starting with Super Mario World 2: Yoshi's Island, where they teach the player controls and give tips rather than saying things outright. Super Mario World 2: Yoshi's Island gives Message Blocks their now-common look of smiling white faces. They reappear in the remake Yoshi's Island: Super Mario Advance 3.
The Message Blocks return once more in Yoshi's Story. Here they fill the exact same role as they did in Super Mario World 2: Yoshi's Island. Their appearance has changed since then, though, as they are now much bigger, and their mouths continually move.
Yoshi's Island DS
They return in Yoshi's Island DS and are much the same as before but return to their Super Mario World 2: Yoshi's Island form.
Yoshi's New Island
Yoshi's Woolly World
They only appear five times in the game, with three in the first level and two in the third. They retain the appearance from Yoshi's Island DS, however their mouths now continually move, like in Yoshi's Story.
Super Smash Bros.
Message Blocks have a cameo in Super Smash Bros.. They appear in the stage Yoshi's Island where one can be seen in the background along with the Super Happy Tree. The same stage also makes a reappearance as playable for the sequel, Super Smash Bros. Melee.
Super Princess Peach
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