Q: How many light bulb jokes does it take to change a light bulb joke?
A: Hmmmm – the probability that a given light bulb joke will be submitted to the net in any given week is .4, and the probability that it will have changed detectably since the last transmission is .2. Hence (assuming independence, which is reasonable since no submitter of a light bulb joke ever seems to know it has been submitted before, within the last 2 or 3 weeks), the probability that it will change in a given week is .08. So it takes about 12.5 light bulb jokes to change a light bulb joke.