There are several Bible verses that compare the influence of things good and bad to leaven. Leaven – or yeast – is designed to make bread rise so it is light and airy. It doesn’t take very much yeast to make two large loaves of bread double or triple in size. In fact, bread recipes often contain salt to mitigate the yeast so bread doesn’t rise too much.
The Bible tells us in Matthew 13:33 and 16:6 that just like yeast, an “influencer” can easily encourage others to do what is wrong or, conversely, to join the Kingdom of God. Explain that this principle is why you are so concerned about with whom they choose to spend the majority of their time.
To illustrate the impact of yeast, find a simple bread recipe that requires yeast. Measure how high the bread is in the bowl both before and after rising. Be careful not to let it rise too long, because it can eventually deflate. Point out that many recipes call for the dough to rise twice for the yeast to have the desired impact on the final loaves of bread.
While waiting for the bread to rise, discuss how they can be “good” yeast and avoid being influenced by “bad” yeast. If you want to dive really deep into the topic, point out that unleavened bread is only considered unleavened bread by devout Jews if it has been completed – from the first mixing of the ingredients to being removed from the oven – in eighteen minutes or less. After that the flour itself begins to develop the tiniest bit of leaven (a variation of how sour dough starter is made). You can even try making unleavened bread in the eighteen minute time frame and then letting one loaf sit out to “rise” for a few hours. Bake both and compare the two finished loaves. Does even that “non” yeast leaven make a difference? Can even tiny bits of negative influence change us in negative ways over time?
Have fun with it, but encourage your children to think carefully about their influence on others and the influence they allow others to have on them.