Hanenbow (pronounced as "Hay-nin-bow") (エレクトロプランクトン, Electroplankton) is an unlockable stage in Super Smash Bros. Brawl. It comes from a music-themed Nintendo DS game, which features extensive interactivity with the microphone; called Electroplankton. The Hanenbow sub-section of this game was also released on its own as Hanenbow on DSiWare for 200 Nintendo Points, or £1.80 on the Nintendo 3DS eShop.
The stage consists of multiple stems with leaves branching out on each stem. It also features the Hanenbows being fired at the stage's platforms, which make a musical note on contact depending on the angle of the platform. The platform's angle, position, and color can be changed by attacking them. Note that the branch on the left can't be stood on (although it serves as the Pokémon Trainer's platform). When all the leaves on the right-most tree are a dark red, a flower sprouts on top of the tree, making another sound effect. This is a key aspect in an event match on the stage, as the flower must be sprouted while avoiding Olimar's attacks as Peach to clear the event and unlock Hanenbow.
This stage is the one of only two in Brawl where a character can't swim in its water, the others being Corneria. This stage is also the only one to have completely interactive music that cannot be altered in the music settings. Individual sound effects can be listened to in the Sound Test, but an overall piece does not exist.
Interestingly, all characters in this stage and the stage itself are in 2D, like on the Flat Zone stages.
 Tournament Legality
This stage is banned in tournaments due to irregular stage design that makes it difficult to traverse, ungrabbable ledges, being overly large, and powerful camping spots, particularly on the bottom left leaf. The sloped leaves can also negatively affect gameplay.
Electroplankton is an interactive music video game published by Nintendo for the Nintendo DS. This game allows the player to interact with animated plankton and create music through one of ten different interfaces. One of these was called 'Hanenbow'.
Within the Hanenbow interface, there were ten more variations of the Hanenbow appearance, with different positions of leaves in each. The Brawl stage Hanenbow merges two of these tiers. The primary platform arrangement in the Brawl stage is a tree growing out of the water, which originates from one tier of the Hanenbow interface, while the other platform arrangement in the Brawl stage is a smaller tree growing from the top of the screen, which is part of a tier featuring three of these, with one tree in the same position as it is in Brawl. There is one more platform in the Brawl stage, which is a singular leaf in the water. The color of this leaf cannot change, and this platform serves as the cannon for the Hanenbow creatures. This leaf is in the same position in all ten tiers of the Hanenbow interface of Electroplankton.
When a Hanenbow hits a leaf in Brawl, it makes a sound just like it did in Electroplankton - the sound is based on the speed of the Hanenbow and the angle of the leaf. The leaves change colors when they are hit either by an attack or by a Hanenbow, while in Electroplankton, only the Hanenbow creatures can change the colour of the leaves.
The leaves change colour progressively from green, to yellow, to orange, and then to red. If the leaves are left without being attacked, their color will revert to its previous state. If all leaves turned red in Electroplankton or the Hanenbow stage in Brawl, a flower would sprout at the top of the central tree.
When the Hanenbow hit the water, they sunk. This is likely the reason characters cannot swim in the stage's water. The stage being completely 2-dimensional in Brawl is likely a reference to the paper-like art style of Electroplankton, or just generally because a sideways leaf would make for a strange platform.