The study conducted three experiments with rats to explore second-order conditioning with food as the unconditioned stimulus. The researchers used a Pavlovian conditioning setup where rat activity was measured. They found that second-order conditioning was clearly demonstrated in Experiment 1. In Experiment 2, removing the first conditioned stimulus did not affect the response to the second one. Experiment 3 showed that both second-order conditioning and conditioned inhibition could occur together. This research sheds light on how animals learn through associations and how different conditioning processes can interact.