From the Super Mario Wiki
“Whack-OOOWWWWWWW!!! Don't hit me!”
Whackas are sentient blue mole-like creatures in the Paper Mario series that live on the west side of Mt. Rugged, near the shore of Keelhaul Key, and near Downtown of Crag. Whacka's name is derived from the popular arcade game "Whack-a-Mole".
If Mario talks to one, they will say that it is a "picture-perfect day" and nothing could go wrong. Hitting one on the head with a Hammer, Shell, or jumping on it will make a Whacka Bump, an edible and valuable item, appear. However, this causes internal injuries and memory loss to the Whacka. If Mario uses Bombette or Bobbery to blast the Whacka, it will be hurt, but will not drop a Bump. If they are attacked eight times, they will run away, releasing a few coins. Since their Bumps are so valuable, Whackas are an endangered species. According to Wonky, there is only one Whacka left in the world; Russ T. backed this claim, attributing the disappearances of the Whackas to hunting. However, in Mario Party 6; on the Snowflake Lake board, several Whackas can be seen popping up out of the snow.
When Whacka disappears in Super Paper Mario, a Cragnon named Norite appears where Whacka once popped out, who claims that the Whacka was her friend.
At one point, the Whacka on Mt. Rugged sends Parakarry a letter stating he one day dreams of flying. The Whacka that appears in Super Paper Mario's Catch Card can be obtained by collecting three hundred shop points. There is also a Sammer Guy by the name of Wrath of Whacka in the game, who says he hopes to avenge the smitten Whackas of the dimensions.
Paper Mario: The Thousand Year Door
Super Paper Mario
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