POW Block

The POW Block is a common block in the Mario series.

''Mario Bros.
The POW Block is an item that appears in Mario Bros. (the arcade version and its ports; not the Game & Watch one). When hit, it damages all enemies in the screen, but only those that were on the ground.

Super Mario Bros. 2
The POW Block has the same function in Super Mario Bros. 2 as it had in Mario Bros., but it can only be used once as the player has to throw it onto the ground to activate it.

A giant version of the POW Block shows up in Super Mario Advance. A Giant POW Block can bounce around the floor, causing multiple tremors, before it disappears.

Super Mario World 2: Yoshi's Island
In Super Mario World 2: Yoshi's Island, if Yoshi hits a POW Block, all nearby enemies turn into stars. It only appears in The Cave of the Lakitus, but appears twice in that one level. They can be hit thrice. POW Blocks were most commonly won in Bonus Challenges, where they could be used anywhere afterwards, being called Anywhere POWs.

Paper Mario
In the Paper Mario series, the POW Block is an item that does two damage to all enemies (but much more in Super Paper Mario if the remote had been shaken with each corresponding prompt). It penetrates defense and makes enemies with shells (such as Koopa Troopas) flip over. In Paper Mario: The Thousand Year Door, the POW Block affects all enemies on the screen, including flying enemies.

Mario Power Tennis
POW Blocks made a small appearance in Mario Power Tennis, on the Mario Classic Court. A POW Block would move back and forth across the net. If a player hits the ball through it, all enemies on that player's side of the court become stunned, allowing the player to defeat them by running into them (This counts as one of two hits needed to stun Sidesteppers; Freezies are defeated instantly).

Mario Kart Arcade GP 2
In Mario Kart Arcade GP 2, the POW Block is one of Waluigi's special items. When used, a random racer is stunned for about 7 seconds.

Mario Kart Wii
The POW Block is a new item in Mario Kart Wii. When dispatched by a player, all racers ahead of the user will find POW Blocks above their karts. They must jump three times with good timing to destroy the POW Block or they will spin out in a similar way to that caused by a Banana Peel. Racers won't be affected by the POW Block if the third strike goes off while they are in mid-air. The player can also avoid it by starting a wheelie, or shaking their controller (like doing a jump trick) right when the third strike goes off. By doing this, they will still spin out and lose items, but retain their speed.

Super Smash Bros.
POW Blocks appeared as part of the hidden stage Mushroom Kingdom in Super Smash Bros. If a fighter hit the POW Block, all other fighters touching the ground would be damaged.

Super Smash Bros. Brawl
The POW Block makes an appearance on the Super Smash Bros. Brawl stage Mario Bros. where it can be used to paralyze all enemies on the screen, allowing them to be picked up and thrown at other players.

Mario Super Sluggers
POW Blocks also appear in Mario Super Sluggers as a item. They are referred to in this game as POW-Balls. The item is an unlockable item that will stun fielding players for about four seconds. They are unlocked when a certain Treasure Chest is opened in Mario Stadium.

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Trivia

 * In Mario Kart Wii, "POW" can be seen as a license plate of one of the cars in Moonview Highway.
 * "POW" also is the liscense plate of numerous cars in Mushroom Bridge and Mushroom City from Mario Kart: Double Dash!!.