A riddle: When is a kilogram that is no longer a kilogram still a kilogram?
Answer: When the chunk of metal that serves as the official international standard for the kilogram, under triple lock-and-key in France since 1889, inexplicably sheds a little weight.
As far as I can tell, all the obvious explanations have been looked at, and the official kilogram is being compared to other kilograms made at the same time in the same way and stored under the same conditions, so it isn't a matter of gravity changing or somesuch.
I don't know if anyone has checked for the shape of the missing 50 micrograms--that might provide a clue, nor do I know what it would take to knock 50 micrograms off platinum.
Sometime before 2010, he said, the mysteriously shrinking cylinder will be replaced with a new-and-improved model, a perfect sphere measuring 93 millimetres in diameter and made of pure silicon-28.
"The advantage of silicon-28 is that it is stable. The mass does not change over time," said Dr Davis.
So if the silicon sphere starts shrinking too, I guess that's more reason to think it's aliens.
Any other examples of intractable weirdness?