Like leebo said I was actually very surprised no one had brought up “a” or “I” until now. It’s a wonderful comparison and not foolish at all.
You can have letters that are both. When you use it as a word though, there’s a set way to read it. When it’s used as a component, there’s a lot more ways to read it.
You can even use “a” to mean “not” like “atypical” or “atheist” but you cant use it as a word to mean “not”. This is similar to something like 不
In western languages there are often words that use different “readings” for components as well, often coming from latin or greek origins.
The word for 水 would be “water” in English.
In compounds we often don’t use the “reading” water, but we might use
the greek “hydro” or the latin “aqua”.
hydrostatics, hydroelectric, hydrocarbon, etc.
aquaculture, aquatic, aquarium, etc.
But there are still words like “waterproof” and “watermark”.
I guess most of those words will be written with 水 in Japanese. Often read すい (like aqua, hydro) and not read みず (like water). But you would not call water “hydro” in English. But “aqua” might be correct or understandable in many languages.
I find this helpful to understand the difference between kanji, readings and vocabulary.
This is a really cool way of looking at it 