Sorry football fans, but this is something a little different than throwing a block on the field. Rather, this refers to a conditioning principle first addressed by Kamin (1969). Kamin indicated that having a CS (conditioned stimulus) that can predict a UCS (also known as US or unconditioned stimulus) is sufficient. What this means is that, if an animal learns that a CS is a reliable predictor of a UCS (e.g., a pigeon learns that a light reliably predicts the onset of some painful stimulus such as a shock), then the pigeon will not become conditioned to another CS or learn that any other CS predicts that UCS. So, our little pigeon friend will not learn that a bell predicts the onset of the shock the same way the light did. Once the pigeon learns one reliable association with the CS, it essentially "blocks" further associations.