Give every member in your class a piece of hard candy (like a Jolly Rancher or Life Saver). Tell them they can't swallow, no matter what.
Continue on with your lesson for about five minutes.
Explain that this is how addiction is. The object of your addiction is constantly on your mind, like the desire to swallow when the candy was in their mouth.
When the candy was in their mouths, that's all they could think about. They probably didn't even notice what was going on around them or hear what was being taught because they were so distracted.
Addictions may not be obvious to those around you, but you can feel them in your mind -- they are a subtle distraction from life. Unlike the candy which will become smaller and eventually dissolve, an addiction will grow and grow until it overpowers your agency. You will need help to overcome it -- from the Bishop/Branch President, counselors, friends, family, and most importantly, the Savior.