Here's an object lesson that you can use to explain how our children inherit a sinful nature, and how it is activated, tempting them to act on their sinful desires and disobey God.

On a table lay out the following ingredients that are used for making bread: a bowl, flour, milk, salt, sugar, a mixing bowl, a jar of warm water, and yeast. Make sure everyone knows what you have on the table.

Then ask the following question: "What is wrong with our children or youth?" Call their attention to the ingredients on your table. Explain that the Bible uses yeast (a single-cell fungus) as a way of speaking about sin, with its ability to permeate. Then demonstrate that in the making of bread, different ingredients go into your bowl. In like manner, when parents conceive a child, they are not just adding the parents' individual DNA; they are also mixing in the yeast of their sinful nature.

Then explain that you must take your dough and place it in a bowl of warm water, cover it, and place it on the stove. The particular environment of warm water has now placed the yeast in an environment where it will activate. You then explain that our children live and grow up in an environment or a culture that the Bible calls "the world." Satan uses the world to activate our children's sinful nature and tempts them to act upon their sinful desires and disobey God.

Check out the video and start at 2:25.