From the Super Mario Wiki
Platform Blocks are objects that appear in New Super Mario Bros. They first start out as a single block with a face on it. Each time Mario hits the block, another Platform Block will pop out above the previous one, creating up to four Platform Blocks piled on one another. This can help Mario reach higher platforms, enter horizontal pipes, or block enemies. However, if Mario takes too long, the blocks will collapse back into a single Platform Block.
The blocks return in Super Mario 3D Land, where the main block is an Exclamation Mark Block. It can conjure more or less than four blocks. The blocks don't always go straight up, they go in some paths as well. The face blocks even have the same design, and the Exclamation Mark Block is sometimes the size of regular blocks or sometimes bigger.
There are certain ? Blocks in Super Mario 3D World that act as Platform Blocks. In the level Super Block Land, there are larger versions of them. They can also be activated by tapping on the Wii U GamePad.
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