"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.