From the Super Mario Wiki
An Egg Block.
Three kinds of Egg Blocks
Egg Blocks are special blocks in the Yoshi series. The most common type is green; however, in Yoshi's Story, they are the color of the Yoshi the player is currently using. When one of the Yoshis hits this block, a Yoshi Egg will come out. Only a maximum of six Yoshi Eggs can come out from the green egg block until Yoshi uses one or more of them, as 6 is the maximum it can hold at once. The one exception to this rule is in the Yoshi's Island Zone DLC for Sonic Lost World, where only three Yoshi Eggs will come out regardless of how many Eggs are currently following Sonic. Yellow and Red Egg Blocks turn into a Yellow Egg and a Red Egg, respectively.
In Yoshi's New Island, two new Egg Block varieties are introduced. One of them releases Mega Eggdozers, while the other one releases Metal Eggdozers. Both of them are four times bigger than a normal Egg Block (looking like green Expansion Blocks). The earlier variety is white, while the latter is black.
In Yoshi's Woolly World, Egg Blocks look more like boxes made out of yarn, matching the game's aesthetics. They serve the same purpose as before, but instead of regular Yoshi Eggs, they instead release Yarn Balls, the game's equivalent of Yoshi Eggs. Occasionnally, they will release bigger Yarn Balls for the Yoshis to use.
- In Super Mario 64 DS, if Yoshi swallows a wooden box, the Egg he creates will be in the shape of a cube, just like Egg Blocks.
- In Super Mario Sunshine, in a secret level in Pinna Park, there are four colored Yoshi Egg Blocks, colored magenta, green, blue, and black.
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