Ask a class member to make a paper airplane, or make one yourself. Tape a coin, rock, or weight to one side of the airplane. Stand on the same side of the room as the class members, and ask a class member to throw the airplane gently toward the other side of the room. Next, pick up the airplane and remove the taped object. Have the class member throw the airplane again. After the class member has done so a few times, put the airplane away, and ask the following questions:
How can just one small weight keep the plane from flying correctly?
Explain that taping a weight to the wing of a paper airplane is like holding a grudge. When we refuse to forgive others, we carry around a weight that keeps us from traveling the straight and narrow path our Father in Heaven wants for us. It is important to forgive others so that we can enjoy the companionship of the Holy Spirit and grow spiritually.
(This object lesson can also be likened to repentance. One small sin can keep us from "flying" back to our Father. We need Christ's atoning sacrifice to make us clean. ~Jenny)
Source: Lesson 34: Forgiving Others, Preparing for Exaltation: Teacher's Manual, 197