we can look across the evolutionary tree and ask which gene in sea urchin goes with which gene in zebrafish, and that goes with which gene in human? and by studying when that gene turns on and turns off during development or in maintenance of a healthy organism -- in sea urchin -- that tells us something about how it might be working in human. >> wow, so genomics, this is a particular example where combinatorics has really helped us. it's allowed us to simplify a problem and it's really given us a purchase on how to do things. but there are actually problems which are combinatorial in nature which we still don't know how to solve, isn't that right? >> well, like charlie, for instance. >> poor charlie, yep. >> he's in what we call a traveling salesman problem. >> yes, he is. >> the idea being you're given a certain number of cities and you want to travel -- starting from one city, you want to travel to all the cities, getting back to where you started in the most economical fashion. now, if there are a small number of cities, the analysis is not difficult. so if charlie only had three cities