I don’t want to seem pretentious, but back in preschool I was a star student. At least when it came to potatoes. I am not sure what it’s called in US preschools, but what we meant by a potato, in my French classroom, was an oval shape in which you put objects. The typical example had two overlapping ovals, one for green things and the other for animals. A green armchair goes in the non-overlapping part of the “green” oval. A lion goes in the non-overlapping part of the “animal” oval. A green frog goes in the intersection. A non-green bus goes outside of both ovals. Etc.
As you probably remember, there are many variations on this, including cases where more than two ovals overlap. The hardest part was when we had to draw the ovals ourselves as opposed to positioning objects in pre-drawn ovals: we had to decide whether to make these ovals overlap or not. Typically they would first be drawn separately until an object that belonged to both would come up, prompting some head-scratching and, hopefully, a redrawing of the boundaries. Some ovals were even entirely contained within a larger oval! Hours of fun! I loved it.
[Side note: meanwhile, of course, the cool kids were punching one another in the face or stealing somebody’s lunch money. But they are now stuck with boring million-dollar-a-year jobs as cosmetic surgeons or Wall Street bankers (respectively) while I enjoy the glamorous occupation of modeling IT systems. Who’s laughing now?]
To a large extent, these potatoes really are all you need to understand about RDFS and OWL classes. OO people, especially, are worried about “multiple inheritance”. But we are not talking about programmatic objects here, in which inheritance brings methods with it. Just about intersecting potatoes. Subclassing is just putting a potato inside another one. Unions and intersections are just misshaped potatoes made by following the contours of existing potatoes. How hard can all that be?
Sure there are these “properties” you’ve heard about, but that’s just adding an arrow to show that the lion is sitting on the armchair. Or eating the frog.
Just don’t bring up the fact that these arrows can themselves be classified inside their own potatoes, or the school bully (Alex Emmel) will get you.