Why unknowns are usually denoted by "X" ? 


More precisely: is [this answer](http://www.ted.com/talks/terry_moore_why_is_x_the_unknown.html) really a serious answer or might be a 1 April joke ? 
Let me sketch it. But please watch it, it is really fun and cool and < 6 min.
(Are there any alternative versions?) 


**Answer - because in Spanish there is no sound for "sh" :)**


Spain was under Arabic influence for a quite a long and played as bridge
translating Arabic knowledge to Europe. In Arabic there was some special
word for unknowns similar to English "something", but it contained sounds "sh"
which was not present in Spanish. It was somehow substituted by "K" and later 
under further translation to Latin it became "X"...



Why unknowns are  usually denoted by "X" ? 
Interesting. Cajori says: "Nor is there historical evidence to support the statement found in Noah Webster's Dictionary, under the letter x, to the effect that 'x was used as an abbreviation of Ar. shei (a thing), something, which, in the Middle Ages, was used to designate the unknown, and was then prevailingly transcribed as xei.'"


The Oxford English Dictionary agrees with Cajori.


