Okay, here's one: how many characters in a science fiction story does it take to change a light bulb?
Give up? Two. One to change the light bulb, and the other to say, "As you know, Fred, the light bulb was invented by Thomas Edison, and operates under the principle of …"
Here's another one, only maybe it's not so funny. How many scientists does it take to change a light bulb? Don't know? About ten thousand to build the Doomsday Device and one to write the report saying that Project Light Bulb is an enormous success and requires further funding.
Didn't like that one? How about this? How many diplomats does it take to change a light bulb? One to lay down an ultimatum over a border dispute, a second to pass on his head of state's threat to employ Project Light Bulb if enemy troops cross that border, and a third to decide that the head of state was bluffing.
Not funny, you say. Well, you're a tough audience. Try this one. How many heads of state does it take to change a light bulb? None. Heads of state don't change anything. Not light bulbs, not their minds, nothing.
Okay, one last joke. How many surviving human beings does it take to change a light bulb?
About a week from now? None at all.