Have students comb their hair, with the stipulation that they cannot bend their elbows. This is quite a hilarious sight to see them trying to comb hair with straight arms. Its basically impossible unless the person has very long hair and can bend over very far. Then give the same comb to a neighbor and again with only straight arms, have the neighbor comb the hair. It is possible for this to happen
You can take this object lesson in several directions. It is the message that there are things we cannot do for ourselves. Another person must do it for us. This can be temple work for the dead, or it can be the savior’s atonement. Develop this idea any way you desire.