Kids might be curious about why, if cigarettes are as harmful as you say, people are still able to buy them in stores and smoke them. Shouldn't something so bad for you be illegal? You can explain that, in some ways, cigarettes are illegal. For example, state laws prevent retailers from selling cigarettes to minors. Many states, and some foreign countries, have public smoking bans in effect for restaurants, public buildings, even some outside areas. Explain that those bans have been passed in recent years because of the negative health effects of smoking -- especially the dangers of second-hand smoke [source: CDC].
You can also give your kids a lesson about the history of regulations and tell them that lawmakers over the years have been making new laws to regulate smoking slowly over time. You might want to explain how tobacco companies once tried to hide the addictive nature of cigarettes from the public. Tell them how ads geared toward kids, as well as flavored cigarettes, have now been outlawed for their protection. You can even appeal to a kid's sense of rebellion against authority if you can convince them the tobacco companies are trying to manipulate kids to smoke [source: American Lung Association].