Take 2 white roses. Make sure they look alike. Hide one so that no one can see. Then take the other one and tell the kids how beautiful the rose is and how good it smells. Have the youth pass the rose around the room. Tell them to smell it, to touch the soft pedels and to see how beautiful it is. While the rose is being passed around give your sermon on purity. At the end of the message ask for the rose back. Almost 100% of the time the rose will come back brown, wilted, and bruised. Show it to the kids. Tell them how everytime you touch a rose’s pedels they get bruised. Just like with sexual impurity we come back no longer white. Then bring out the other white rose and say that when you give all of that impurity to God that He makes you white again.