Another thing I understood yesterday (somewhat) is how injective and surjective functions are related. This has been bugging me since high school -- they seemed

Another thing I understood yesterday (somewhat) is how injective and surjective functions are related. This has been bugging me since high school -- they seemed "too different to be opposites", but they should've been opposites. CT has a very simple graphical representation of how the two are, in fact, opposites and my life is richer for it :-)