The following is certainly not a number trick but I thought is was cute. There are probably thousands of pages on the www devoted to number tricks and riddles of all sorts. Often they are sent to me by readers from all over: north, east, west, south (did you know that "news" is a word indicating all four directions?) ... Anyway, here's a short riddle that tickled my fancy:What occurs once in a second, once in a minute, twice in a millennium, but never in a lifetime? ... Give it some thought and then scroll down.
The letter N ... how can you not know; it's obvious.
Seriously, using some advanced statistical analysis with a bit of calculus thrown in, I'd say that 10 out of 9 people wouldn't know the answer... I don't know, maybe even more.
And finally, there's what we call "undecimal counting" (that's counting by 11). Weird things happen with 11. For example, in multiplying by eleven ... 111,111,111 x 111,111,111 equals 12,345,678,987,654,321.
An interesting extension of this undecimal counting is seen in the ISBN number (International Book Numbering System) of published books. Any ISBN comprises ten digits. Now here's the thing; if you multiply the first digit (on the left) by ten, the second by nine, the third by eight, and so on, summing the results as you go along, the result will always be divisible by eleven. This is one way of checking to see if an ISBN is legitimate; it doesn't work with any 10 digits taken at random.