I think that most people who look inside the tank and spend a few seconds analysing what happens when you flush have a pretty good idea how that part works. But it's the other end that's more opaque (literally.) What makes the bottom half of the toilet work, where the real magic happens? I think that's the part that most people would struggle with since few of us have transparent toilets and the internal structure isn't always obvious from the external shape of the stool.
This explains how the siphon works to keep the bowl at the same level and to create the force of a "flush". Until I saw a toilet that had the siphon shape visibly exposed on the outside, I don't think I ever thought question how it worked. Once I saw the shape, I immediately thought about it working like a siphon I'd used to pull gas out of my car. For people who have never seen a toilet with the siphon shape exposed, they'd probably have to have siphon experience in some other area to make the leap, or I think how the bowl stays full and flushes would just naturally stay a mystery. Believing that it all happens in the tank might also be enough of an explanation for most to stop thinking about it.
As someone who just suffered through a clog recently, I liked this explanation from an MIT student better. It starts out the same, with a good diagram and a description of the basic principle, but then goes the extra mile with actual physic equations.
