Nessa's Gym Mission is a water pipe puzzle. You'll need to press buttons to change the direction of water flow, aiming to make the water reach a specific point. There are three sets of pipes, each with a different solution. Pay attention to the arrows indicating the water's path. If you make a mistake, you can reset the puzzle. It's a logical challenge that requires a bit of trial and error. Successfully completing it will grant you access to Nessa.
