From the Super Mario Wiki
The Aurora Block is a powerful, rainbow-colored block used in Mario & Luigi: Partners in Time. It is notable for its ability to grow to match the size of the heart of the person who strikes it, and its very existence is celebrated in the Star Realm.
During the events of Mario & Luigi: Partners in Time, Mario, Luigi, Baby Mario, and Baby Luigi find a talking gate at the Star Shrine who tells them that they cannot continue unless he "reads" them to see if they're worthy or not. Although courageous Mario passes the gate's test easily, chicken-hearted Luigi is not as fortunate. The star gate gives them three options: go home and eat some pasta puttanesca, continue without Luigi, or find the elusive Aurora Block.
The team quickly chooses the third option and sets off to find the ominous Block. The quartet discovers it quite easily, but Luigi finds himself unable to hit it, with the block moving out of the way whenever he tries. The gate then asks Luigi who did the work of finding it: Mario, the babies, or Luigi himself. Ironically, the correct answer was the whole team, which was not a choice. Luigi tries reasoning with the gate, but it talks back by calling Luigi a liar and continues to sass him until he starts crying. Mario then bluntly claims Luigi was telling the truth.
After that, the Star Gate finally comes clean and reveals that this was all just a test to measure their brotherly togetherness. He then gives them the titles of "best siblings ever" and tells them that they're all ready to proceed to the rest of the Star Shrine. Best of all, the gate lets the confused Luigi hit the Aurora Block, which he does hastily. However, it is then that the gate chooses to tell Luigi that the Aurora Block can grow to the size of the heart of the person who hit it, and, after he says this, the block turns gigantic and comically flattens Luigi (who now resembles Baby Luigi in Baby-cake form). Baby Mario picks the pancake plumber up, and the Star Gate opens.