From the Super Mario Wiki
A Glow Block (Light Block in European versions) is an item found in the game New Super Mario Bros. Wii and New Super Mario Bros. U. As the name suggests, these white blocks emit light that Mario and the other characters can use to illuminate dark areas. The characters must carry them in order to explore different areas with the help of the block's light. To pick up a Glow Block, the player must shake the and press and hold at the same time. Glow Blocks are only present in dark and murky areas such as caves or Ghost Houses. They can also be used as a projectile, by throwing it to the enemies, in similar fashion to Super Mario Bros. 2's physics.
In New Super Mario Bros. U, these blocks can not be seized as in the Wii version and only act as platforms but still illuminate dark areas and some are activated by pressing a ? Switch. They widely appear in Light Blocks, Dark Tower.
In addition to normal-sized Glow Blocks, bigger moving varieties also appear. These blocks appear as moving nearly-invisible blocks at first (which look similar to Blocks from Super Mario World), but if Mario or any other character hits a nearby ? Switch, the blocks temporarily glow and keep moving. These blocks can be used for both lighting the way and means of transportation.
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