But if for some reason you find you must, never explain the meaning to them. This is one of the most effective ways of convincing them that Christianity is merely a tradition and not the Truth. Don't warn them about evolution or other popular myths. Don't inform them about heresies in history or their modern iterations. Tell them nothing about competing theologies and why orthodox churches reject them.
Fads are there to be followed.
If they want tattoos or multiple piercings, sit back and play the fat, passive idiot. After all, it's their life. And don't even look at what they read. They have rights, you know. Don't make them work for anything.