An important component of addiction to a drug such as morphine or heroin is tolerance; that is$,$ the user requires greater and greater doses to experience the same effect. Shepard Siegel hypothesized that this tolerance applies primarily in the context $($such as a particular time and environment$)$ in which the doses were given. If the user receives the drug in a different context, tolerance will not apply and the user will experience the full effect of the dose. With a drug such as heroin, the result may be a fatal overdose.

To test this hypothesis, two groups of mice are made morphine$-$tolerant by repeated doses in a training condition. Then one group receives a standard dose of morphine in the same environment as in the training condition. The other group receives the same dose in a new environment, the testing condition. After receiving the morphine, each mouse is placed on an uncomfortably warm surface, and the time until the mouse licks its paws for