Cats received a fish reward whenever they maneuvered themselves out of an enclosed puzzle box. with successive trials, the cats escaped from the box with increasing speed. this illustrates:

This illustrates the law of effect.
Accoding to the law of effect. responses that resulted in satisfyin effect for the doers will provoke a similar situation to happen again in the future.
In this particular case, since the reward provide satisfying effect for the cats, the cats will more likely to repeat the same manuever under similar situation.