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Bones automatically runs through the level, trying to finally find some rest. He can be black or white: You can change the color of Bones by pressing Space or Y on your controller.
In order to be able to interact with level elements such as fences, jump pads or levers, Bones has to have the opposite color. If he has the same color as a level element, he will ignore it, causing him to fall through fences or to miss levers. But in some cases exactly this behavior is required to progress, which allows some tricky level design.
The goal is to reach the end of the level.