A group of astronauts are on the moon. They've been mining the surface, and have discovered that it really is made of cheese.
One particular area of cheese that they're quite interested in is a large vein of brie, and they've already been there twice, and collected samples to be returned to mission control.
All of a sudden, the radio crackles into life: "Mission control to cheese-base-one - we need you to get a third load of that brie!"
But the astronauts are unhappy with the idea. They try to come up with all sorts of excuses why they shouldn't dig any more...
"It'll spoil the environment if we take too much. We don't want to leave this place looking bad. After all - have you ever seen such a site in your life as brie mined thrice?"