You get a rat and you put it in a cage, and you give it two water bottles: One is just water, and the other is water laced with either heroin or cocaine.
Duodi du buteliukus vandens. Viename paprastas vanduo, kitame –  vanduo su trupučiu heroino arba kokaino. Taip padarius, žiurkė beveik visada  pasirinks narkotinį vandenį.