A rat will not press a lever it has never touched. So trainers use shaping. They reward successive approximations, or steps that get closer and closer to the goal. Here, you set the rule for when the animal earns a treat. Start easy, then make the rule stricter, one step at a time. If the rule is too strict too soon, the rewards stop and the behavior falls apart.
The training box
Reward rule: give a treat if the animal ends up within-