According to Thorndike, learning occurs via the association between the sensations and the impulse (action). As for checking his theory, he created puzzle boxes for the animals that he used for his experiments. He put the animals into the boxes and measured the time they spent for learning to open the door. He discovered that animals do not have enough ability to figure out the results of their actions. They find the solution with trial and error process and mostly by chance. After finding the solution (that is the solution that leads to the correct impulse, namely escaping the door), the time for operating the correct response to a specific situation decreases. However, when the reinforcement is removed (in this experiment, escaping the door is prevented), then the corresponding sensation (attempt to pull down the lever) disappears (This event can remind us the term "extinction" used by Pavlov in his classical conditioning theory). After introducing the reinforcement again, sensation reoccurs but the animal needs time to make the escaping operation more quickly again as he learnt before removing the stimuli.