From the Super Mario Wiki
standing on a Rolling Crate
A wooden Rolling Crate (left) in Super Smash Bros. for Wii U
A Rolling Crate is an item featured in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U. Although it is technically a Crate, its effects greatly differ. Once hit, instead of being destroyed and expelling items, the Rolling Crate will begin to travel about on its wheels. It will run over any player in the way, damaging them. Players can stand on top of it as it rolls by, unlike regular Crates. If attacked with enough force, they can be broken open to release items.
| Rolling Crates
|| Wii Super Smash Bros. Brawl
|| Special crates that look like normal ones but have four wheels attached to the undersides. Try to destroy these and they will go rolling along, smashing everyone in their path. When a rolling crate is destroyed, it will reveal the same sorts of items found in normal crates. You can also jump on a rolling crate for a fun ride.
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