It seems to me that there is a predictable life cycle of the ubiquitous email forward. Let's say you receive this "hilarious" email forward about how the mouse cursor moves around the screen, or how a power line worker inspects the distribution lines.
Email forward duplicates seem to be like ripples on a pond. At first they're closely spaced together. Then, as time passes, they become more spaced out. I'm sure there's some formula with the longevity of the cycle being directly related to the initial frequency of forwards you get for that message. Now if only I could find a formula to predict the span of time between each time the message is sent to me.
Also, this is not an invitation to send me any forwards. Only to see if you notice a pattern in your inbox.