n-back is a method to test your short term memory.  According to some research, it may increase your fluid intelligence.  I’ve been playing with BrainWorkshop, a neat little open source program  for n-back.  It works, I’ve already become smarter.  My proof is that I invented a word puzzle.
The n-back value of a word is the number of letters repeated at the nth position, for example the 2-back value of dad is 1, because the last d is two positions from the first d.  A letter in bold shows a repetition.  The 2-back value of my name, Adam, is also 1.  Higher numbers can also play, the 3-back value of member is 1, while its 2-back value is also 1 (member).  Is 1 the highest n-back value for words?  No.  Some of my favorite ones, nonsense, contented, stomachache all have 3 as their 3-back value.  Or the simplest one, tartar, which is nothing else but repetition.  They look easy, because its a whole block of letters that are repeated.  But there are trickier ones, like fulfilling where the repeating letters are interspersed.  Which English word do you think has the highest n-back value?  I don’t know.