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Mountennis is a downloadable microgame in WarioWare: D.I.Y. and WarioWare: D.I.Y. Showcase. It was released on October 7, 2009 in Japan, September 6, 2010 in North America, and October 8, 2010 in Europe.
At the start of the game, the player sees a climber and tennis player atop two mountain peaks, sometimes with snow. The climber will yell Yahoo or Yahooo. The tennis player needs to hit these words back. The game keeps score, once per game, but is ultimately for cosmetic purposes.