A joke

At university, I shared room with three other classmates. We all had the habit of smoking. Nonetheless, the only person who bought cigarettes was I. Every time when I bought a pack of cigarettes, It would be consumed within one day. Therefore, I gradually reduced the frequency of buying cigarettes. As a result, no one smoked any more. We all quitted smoking. It was not until we graduated from university that I found out the fact that they didn't have the habit of smoking. To get me rid of smoking, They came up with this idea!