Each one takes turns winding up the other so that they can continue their adventures. The closest of friends, Salt is a witty wise cracking narcissist, whereas Pepper enjoys talking about politics and movies, instead of disliking each other for these differences they embrace them.
If either of them runs out of energy, they can no longer function until they are wound up using the key on their backs. This means that most of their friendship is spend simply winding up the other, so that they, in turn, will be wound up afterwards. ~ via Alex G