I ran across this little blurb in the 10/27/08 issue of BusinessWeek that I found endlessly interesting.
People, apparently differ radically on what makes things similar (from a study in Cognition). As a marketer, this really is fascinating. The study showed that about 50% of ~200 subjects defined similarly in a "categorical way," lumping does with cats because both are animals. The other half though more thematically, meaning that they paired dogs with bones instead for example.
The blurb points out a lesson for marketers: if you sell cake online, include "milk' in the search terms you buy!
The human mind continues to fascinate.