A lecturer on stress management raised a glass of water and asked his audience, "How heavy is this glass of water?" Various answers were called out. Then he replied, "The weight really doesn't matter. What matters is how long you try to hold it.

"If I hold it for a minute, that's not a problem," he said. "If I hold it for an hour, I'll have an ache in my right arm. If I hold it for a day, you…