From the Super Mario Wiki
A Bros. Block is a special block found in Mario & Luigi: Superstar Saga and Mario & Luigi: Dream Team. It is a yellow block with an arrow on it. When struck by either Mario or Luigi, it will move in the direction of the arrow and change to the opposite brother's color and initial. When struck by that brother, it will move back to where it was and change again. Striking Bros. Blocks take speedy timing, as there are usually twenty coins in each one. However, if the brothers are too slow on their jumping, less coins will be earned.
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