One of the most common mistakes that Christians make today is that they carry their emotions on their sleeves. When someone wrongs us or says something hurtful, we hold on to it and allow the anger to build a Spirit of Offense. When we do this and don't forgive them, we suffer in our relationship with God.
To get this message across to students, have a leader put on a long raincoat with a hood (make sure it reaches knees or lower). Begin to slowly pour – drop by drop – some water onto their head. As the kids will see, the water rolls down the jacket without getting the person wet. Drive home the point that when we allow offense to build up in our lives, it acts as a raincoat, spiritually. It makes it difficult to receive anything from God.
Then tell them that when they ask for forgiveness for themselves and the person who offended them, it removes that Spirit of Offense so they can again receive from God. Show them this by having the leader remove the raincoat then pour water (again, drop by drop) on them, showing that now, with the offense removed, they can receive from God.
Idea by Ryan Kramer