Kamin’s famous blocking experiments (1969) showed that not all contiguous structures lead to classical conditioning. A rat that has already learned that CS1 predicts a US, will not learn that a subsequent CS2 predicts the US, if the CS2 is always paired with the CS1. Suppose that a rat has learned that a light predicts a shock because of the constant contiguity of the light and shock. After learning this, the rat has a sound introduced which only arises in conjunction with the light and the shock. As long as the rat had previously learned that the light predicts the shock, it will not learn that the sound does (as can be seen on later trials that have the sound alone). In sum, having learned that the CS1 predicts the US blocks the organism from learning that the CS2 predicts the  US.[34] So even though CS2 is perfectly contiguous with the US, the association between CS2 and the US remains unlearned, thus serving as a counterexample to sufficiency of  contiguity.[35]