Cumulative Record. Chapter 46: How to Teach Animals. Quote 8

The best way to reinforce the behavior with the necessary speed is to use a “conditioned” reinforcer …. For a conditioned reinforcer you need a clear signal which can be given instantly and to which the subject is sure to respond …. A convenient signal is a rap on a table with a small hard object or the noise of a high pitched device such as a “cricket.” (p. 606)