matthen:

Pick a number between 1 and 99, write it as a word, then count the letters to get a new number. Write the new number as a word, and count its letters- repeat.  This curiosity was tweeted by @IanMathmogician, and if you try it you’ll find you get stuck at the number 4- the only number equal to its letter-count.  This image shows the network of what  your possible routes to 4 could be. It shows you’ll always get there after 5 steps. [code] [French version] [full size image]

matthen:

A quick maths mind reading trick you can do.  It works because abc minus cba is always a multiple of 99.