The Key is an item found in various games. Whatever they open varies and depends on what the character needs it for.
Fatty Bear will need the key to the attic to gain access to the attic. In order for the mouse to give you the key without going back in with it, Fatty Bear will need to give the mouse some cheese (which he can get from the fridge) so that it will leave the key behind. After the player gives the cheese to the mouse, Fatty Bear will get the key and use it to unlock the door.
Doing this is optional unless the dog takes away the chocolate chips and it rolls in through the hole inside the door.
The key is one of the five items needed to make the rocket. Putt-Putt can find the five keys at the building at the top of the hill next to the rocket. The keys always vary in shapes and colors, and are only given out to good-deed doers by Governor Moonbeam. After Putt-Putt saves an alien from moon goo, he is allowed to pick a key. If he selects a key and it isn't highlighted on the blue flag, it's because he chose the wrong key, so he has to pick the one whose shape matches that on the blue flag. After Freddi gets the nose cone, steering wheel, and the rocket fuel in the right places, he will need to use the key to start the rocket ship.
Freddi and Luther will need the key in order to release Fiddler Crab from his cage. Freddi Fish and Luther will find the key where Ray and the oysters are. After Freddi and Luther use the key to release Fiddler Crab from his cage, he will give the duo his fishing pole as a reward for saving him.
This is optional unless the bottle is at the Deep Canyon and it dropped into the home of the angler fish