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There was something else I wanted to mention about intervals. Obviously there are two competing systems. The traditional system goes back centuries. I think your idea of talking about trees is a good explanation of how it works.
If you have three trees and you want to talk about how much space they take up, you can put a rope touching the tree on the left and stretching to the tree on the right plus the tree in the middle. That would be a very good description of the traditional concept. We can then talk about the whole thing with this in mind. And that describes what we have with a third when we talk about having three letters. Then we sort of refine it by talking about the absolute distance from one to two to three, again talking about those trees.
When we talk about a diminished 3rd, it might be like keeping those three trees in mind but decreasing the distance between those trees so they become narrower together. This may be forcing the analogy, so I'll let other people decide whether any of this works for adults.
Regardless, in my own teaching I call this system letter intervals. It doesn't describe what we hear, only what we see on a page. We can't quite say that a minor 3rd and an augmented second is exactly the same thing, although if they are not exactly the same thing they are very, very close in sound. And on the piano they are exactly the same.
The more modern system and the one I always use could be called the sound interval system. And this system only cares about what we hear. So if we redefine a minor third as one and a half tones, and we redefine an augmented second as one and a half tones, we are free to either consider them as exactly the same thing, or slightly change the tuning if we are thinking about an older tuning system that is not equal temperament.
Some of these intervals do not have universal sound names. For instance, we have a tone, and we have a tritone. Unfortunately the name "bitone" does not exist. But that interval would describe the distance of two tones. And it plugs a gap.
In contrast, the term tritone not only exists but is used universally. And for that interval, when we are listening to it, we don't care if it is an augmented 4th or a diminished 5th. We simply hear that it is slightly bigger than a 4th or slightly smaller than a fifth. We then play what we hear or make a piece of music with that interval. Then if we notate it, we decide which look is more appropriate. For instance, if you are putting this interval into context and your tritone is opening up to a fifth, it is likely that you will play that tritone a bit sharper. If it is shrinking to a 4th you will play that tritone a bit flatter. You make the adjustment according to what you hear, either based on theory, or based on your preference.