- a ream of paper
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. (You might write the word action or choice on the paper. It's easy to rip up or stop performing a single action. - Jenny)
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. (Compare this to repeatedly making the same decision until it becomes a habit. Habits can be good or bad. Either way they are hard to break. - Jenny)
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.