Using the new camera on the Hubble Space Telescope, astronomers have been able to confirm that the Moon is made of green cheese. The telling clue was the resolution of a marked date after which the Moon may go bad.
In the interest of full disclosure, I include the last line of the picture explanation. ;)
It has been used historically in context to indicate a claim so clearly false that no one -- not even April Fools -- will believe it.--MissM
Astronomy Picture of the Day