One of the social workers in my department sent out one of those "amazing" math puzzles, that tells you to multiply by 5, add 20, etc, and the result is a number that reveals your age and some other original number you started with. They always say breathlessly "This is the only year it will work!"
She sent it to the whole department - 30 people - asking if anyone could explain how this amazing math stunt worked. I have never found these particularly interesting. The principle is obvious: you you add and multiply stuff up, and you subtract and divide the same numbers back down, just disguised. To take an over-simple example, you add 9 and 2 at the beginning, then subtract 6 and 5 later on. 11 up, 11 down.
I don't think I had ever bothered to take one apart completely, though I may have years ago. But I thought it my duty to fight innumeracy in the social sciences, so I made an effort to explain the solution in as simple a way as possible. I pointed to the unusual number 1757 that was stuck into this whole mess and noted it was a nice round 250 from the current year. I reminded folks that the year of their birth, which was asked for in the problem, plus their age always equals the current year. It's a trick, folks. They got you to reveal you age via your year of birth. Duh.
It "only works this year" because last year 1756 worked, and next year 1758 will work. Shazaam.
I was mildly distressed at the people who were so grateful and impressed that I had explained it to them. Sheesh. Then later I heard about the people who still couldn't follow it. CP Snow's Three Cultures, extended out 40 years later. Ouch.