From the Super Mario Wiki
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Shell Blocks are blocks that appear in Mario & Luigi: Bowser's Inside Story and its remake. Striking one allows Mario to wear a blue shell and Luigi can then knock him forward using his hammer. This block is made available when Mario and Luigi meet the Koopa Paratroopa Trio. During their travels, Mario and Luigi find a giant Shell Block above the clouds in west Toad Town and strike it, spreading several normal-sized Shell Blocks around the Mushroom Kingdom. The Koopa Paratroopa Trio notices this and begin yelling at the two. Eventually, they give the two bros a block to use. Mario strikes the block and a shell comes out and lands on Mario's back. After using it, the Koopa Paratroopa Trio sees that the Mario Bros. can use the shell quite well and teaches them the proper way to execute it. Once they learn correctly, the Koopa Paratroopa Trio flies off and gives the two the ten Toad Town Attack Pieces for the Yoo Who Cannon. The blocks are scattered around the over world and inside Bowser for Mario and Luigi to use.
If the player attacks an enemy with the shell power-up in the field, the enemy will be temporarily stunned.