The Rescorla-Wagner model argues that there is a limit to the amount of conditioning that can occur in the pairing of two stimuli. One determinant of this limit is the nature of the US. For example: pairing a bell with a juicy steak is more likely to produce salivation than pairing the bell with a piece of dry bread, and dry bread is likely to work better than a piece of cardboard. A key idea behind the R–W model is that a CS signals or predicts the US. One might say that before conditioning, the subject is surprised by the US. However, after conditioning, the subject is no longer surprised, because the CS predicts the coming of the US. (Note that the model can be described mathematically and that words like predict, surprise, and expect are only used to help explain the model.) Here the workings of the model are illustrated with brief accounts of acquisition, extinction, and blocking. The model also predicts a number of other phenomena, see main article on the model.