We tend to teach children and teens about sin as if each sin is an isolated incident that is self contained. While sometimes sins can be isolated incidents, many times they have a ripple effect that can lead to more and more sins and even a life mired in sin. You can teach a didactic lesson about one lie leading to other lies or one sin being covered by lies and then more sins, but sometimes a visual object lesson has a bigger impact.
If you are not familiar with object lessons, they were often little science experiments of sorts that illustrated a spiritual lesson. Think of them as a sort of visual parable. There is a great object lesson to illustrate how sins can quickly multiple and seem to spin out of control.
Place children or teens in groups of two or three so they can easily see what happens. Give each group a bowl of really warm water, a little yeast (preferably rapid rise) and a teaspoon of sugar. Have them mix stir the sugar into the water and sprinkle the yeast on top (it’s easier to see the expansion if it isn’t mixed into the water). They may need to use magnifying glasses to see the yeast multiplying. If you want to add to the visual make some dough with yeast and some without a couple of hours before class so the students can see the impact yeast can have as it multiplies.
If you have older students, you can also give them some salt and instructions to stir it into the rising yeast. It should stop the growth quickly (FYI that’s why bread recipes have salt… without it the rising of the bread can get out of control!).
Afterwards talk about how sin can be like yeast. Ask them to think of examples of how one sin can make it tempting to add more of the same sin or additional sins. What are ways that they can stop sin in its tracks, before it spins out of control? Have fun with it, but make sure they understand that sin can quickly multiply and spin out of control if they don’t truly repent.