Operant conditioning, sometimes called "instrumental learning", was first studied by Edward L. Thorndike (1874–1949). He observed the behavior of cats trying to escape from home-made puzzle boxes. When first put in the boxes, cats took a long time to escape. With experience, successful responses occurred more frequently, enabling the cats to escape in less time. In his law of effect, Thorndike theorized that behaviours followed by satisfying consequences tend to be repeated, and those that produce unpleasant consequences are less likely to be repeated. In short, some consequences "strengthened" behavior and some consequences "weakened" behavior. Thorndike produced the first known learning curves by this procedure. 