This object lesson works best for senior primary aged kids through adults. Choose a member to help you. I usually always choose a gentleman who looks like he is fairly strong. Hand him one sheet of paper and ask him to rip it in two. Obviously he will be able to accomplish it easily. Then hand him the ream (or several sheets of paper) and ask him to rip the ream into two. Unless he has Herculean strength he will not be able to. If time allows I will slowly add to the stack and ask him to rip. He generally has an easy go until the stack gets bigger.