I have been working my way through Aaron Hillegass' book Cocoa Programming for Mac OS X. In the second edition of the book on page 135 he has a challenge as thus:

"Make the application sort people based on the number of characters in their name. You can complete this challenge only Interface Builder -- the trick is to use key path. (Hint: Strings have a length method.)"

I am reading the book but using a newer version of Interface Builder. So sometimes not everything is precise mapping. Which is probably just as well because it forces me to clearly understand the concept and not just copy by rote in the book. But the above challenge stumped me for a little bit. It wasn't obvious what should be changed. But figured it out. The trick is to set the "Sort Key" field with the instance var + length property. See attached.