From the Super Mario Wiki
Mario and Luigi hitting the 99 Block
The 99 Block is a special block in Mario & Luigi: Bowser's Inside Story. It can be accessed by traveling to the Challenge Node in Bowser's body. The 99 Block can be used after Mario and Luigi gain access to Toad Town. The initial appearance of the 99 Block resembles a regular ? Block, but it turns into an M Block or an L Block as soon as it is activated. Once this has happened, the Mario Bros. have to take turns hitting it. Each hit causes the block to speed up. If the block is hit 99 times, the player can double the Coins in possession of the Mario Bros. and Bowser when they want. They can get their coins doubled only once. If the Mario Bros. do not get 99 hits, a nearby Emoglobin records the highest score.