A great object lesson you could use to discuss purity.
Show your group a beautiful jug or other container. Describe to them its beauty, worth, etc. After all it can hold many delicious drinks. Pour some yummy juice in it. (Maybe orange juice or soda- something appealing). Find out who wants a drink of the juice/soda and why. Give the young person a drink and ask him/her how it was.
Now add one tiny teaspoon of sand, mud or any other disgusting item (some people actually add dog poop!) to the juice and now offer the same young person the drink. (Obviously don't let them drink it- most likely, he/she and the others will refuse to drink it). Emphasize how good the drink is and what other delicious substances are in the juice and how small an amount of the dirty substance is present in it. Most young people would refuse to drink it especially if the substance is something as disgusting as dog poop.
We have things in our lives that are not pleasing to God. (Gossip, hatred, bitterness, sexual impurity, etc.) In the same way that we would not want to drink a beverage with an impurity in it, despite whatever good things it was made of, we should not expect to live complacent Christian lives, mixed with our goodness and our unholy habits.
We are not perfect, but our desire should be to be purified each day by the Holy Spirit and to let him get rid of the impure stuff in our lives - because our God deserves only the best!
Added by Janine Jack
Also see Sin-Special Brownies