In 1942, a researcher named Liddell trained a sheep to lift its leg off a shock pad to avoid getting shocked when it heard a tone. He was curious to see whether the sheep made a stimulus-response association or a stimulus-stimulus association. To test which association the sheep made, he turned the sheep upside down and rather than its leg on the shock pad, he placed the sheep’s head on the shock pad. When he presented a tone, the sheep lifted its head! What type of association did the sheep make?