Invisible World of the No. 7:The 7 deadly sins, the 7 continents, the 7 seas: The no. 7 is everywhere. But is its prevalence simply a byproduct of the brain's pattern recognition machinery, does the no. 7 play a significant role in both human psychology and physiology - and to what extent is one driving the other? Find out in this Stuff to Blow Your Mind episode.