Another random crawl through the web brought me to this interesting fact. It seems it really is a big riddle to find a solution for. I mean look at the riddle. It does ring a bell for an ardent Hindi movie buff. (won't call it Bollywood!!! Though am not sure why, but as they say some things are better unexplained. May be a bit Patriotic feeling, which pops up every now and then).

A famous dialog from the movie “Chupke Chupke” starring Dharmendra and Amitabh. “Gende Ka Ful Ful Ho kar bhi ful kyun nahi hain?” starts guffawing into the ear.

By the way here goes the main story:

"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.Very little, though. The cylinder of platinum and iridium, sitting under three concentric glass bells like a metallic round of goat’s cheese, is 50 micrograms lighter than the average of several dozen copies."

Weighty mystery: kilo standard sheds 50 micrograms - ABC News (Australian Broadcasting Corporation)