From SmashWiki, the Super Smash Bros. wiki
Boomerangs first appeared in Super Mario Bros. 3 as the Boomerang Bro.'s weapon of choice. A Boomerang item was included in the game's remake for the Game Boy Advance, Super Mario Advance 4: Super Mario Bros. 3. This boomerang could be caught and reused, much like the one in SSB4. However, it could only be accessed through a special e-Reader card released only in Japan. The Boomerang Flower is a power-up item first appearing in Super Mario 3D Land, where it transforms Mario into Boomerang Mario, who is able to throw boomerangs to attack enemies, similar to the Boomerang Bros. enemies. The boomerang would also collect any item it touches, allowing Mario to pick up far-off coins and other collectibles. It also appears in Super Mario 3D World and Mario Kart 8.
The Boomerang can be thrown to attack other players. After reaching a set maximum distance, it homes on its thrower and is automatically caught on contact: whenever this happens, the Boomerang becomes more powerful. However, it can also be caught by opponents.
When thrown fresh, it deals 2 points of damage coming out and 1 point coming back. Once caught, its damage output adds 2 points for coming out and 1 point coming back in and maxes out after 6 catches, putting its max damage output at 14 points coming out and 7 coming back.
The boomerang does not need to hit an opponent for the multiplier to occur, but it does require the boomerang to return from a full throw without hitting any solid surfaces. The boomerang will drop if thrown into the floor from a standing position or if it hits something solid on its return trip and must be picked up again. In any case of the item dropping, its multiplier will reset.
Boomerang Mario would use this item to collect faraway items and defeat distant enemies. In Smash Bros., it can be thrown up, down, left, or right. If you throw it, you can catch it again on the return trip. The boomerang gains power with each throw, so try to keep catching it!
As thrown by Boomerang Mario! He used this to hit enemies and collect faraway items. Here, though, you can throw it up, down or to the side, then touch it when it flies back to catch it. Simple! And as long as you don't drop it, it'll get more and more powerful with every throw, so make sure you do catch it, all right?
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