Thinkage [updated]
"If you had asked me what causes drug addiction at the start, I would have looked at you as if you were an idiot, and said: 'Drugs. Duh.' It's not difficult to grasp. I thought I had seen it in my own life. We can all explain it. Imagine if you and I and the next twenty people to pass us on the street take a really potent drug for twenty days. There are strong chemical hooks in these drugs, so if we stopped on day twenty-one, our bodies would need the chemical. We would have a ferocious craving. We would be addicted. That's what addiction means. "One of the ways this theory was first established is through rat experiments -- ones that were injected into the American psyche in the 1980s, in a famous advert by the Partnership for a Drug-Free America. You may remember it. The experiment is simple. Put a rat in a cage, alone, with two water bottles. One is just water. The other is water laced with heroin or cocaine. Almost every time you run this experiment, the rat will become obsessed with the drugged water, and keep coming back for more and more, until it kills itself.