Being IN the World, but not OF the World
Take a fairly transparent bowl filled with clean water, slip in a zip-lock bag with clean water, show the students the side of the bowl, but make no mention of the zip-lock bag.
Then dump in dirt, food coloring, grass, pencil shavings, etc. Put a lid on the bowl and shake to mix. The bowl should appear well discolored and dirty.
Then take off the lid and pull out the zip-lock bag. It will have remained clean and pure.
In an environment of dirt and filth, one can remain clean and uncontaminated, "in the world but not of the world," or found standing in holy places, nurturing the Spirit of God rather than the spirit of the world.
Ask: "Why is the water in the bag still clear?"
(It was sealed—Members of the Church are sealed as families with protective ordinances and covenants. Nothing got into the bag. Keeping our eyes single to God will keep the world's dirt out.)
Show jar of bottled fruit.
We work hard to create this nice product.
Then what do we do?
We seal it.
Why?
To keep it safe and pure and uncontaminated and to help it last longer.
It is the same with our Families. We work hard to create our wonderful family. Then we Seal our Family to help protect it from the contamination of the world and to make it last.