2 kinds of nails, straight and bent. You also need a piece of board without any imprints.
Take the kids and the nails out back. Tell them you are going to talk about decisions.
Tell them the straight nails are for the good decisions they make every day (i.e., for brushing their teeth, doing their homework, and developing good healthy relationships). Then have them put several nails in the board.
Then tell them the bent nails are for bad decisions (i.e., smoking, drugs, tantrums, cursing, poor attitude, delinquency). Have them hammer a few of those in the board too.
Then you have them give you examples of each.
Then you have them take the bent nails out of the board and look at the holes that are left behind... not just the nail holes, but the hammer marks and other damge too.
Then you say:
"You see, each decision, makes it's mark on your life. Both the good and the bad leaves its impression on your future even long after the decision is gone and forgotten. You can remove the bad decisions and even gain forgiveness for poor decisions and actions, but the mark will remain.
Remember that as you make your decisions."