There is a lot of abstraction to work through, but referring to rule 4 it isn't for actual 5 year olds.

I suppose, if I were to try, I would say that files are written like you might write information in wet concrete that never dries. The write operations is taxing, you have to sit there with a stick and etch in information. At the beginning and ending of each block of etchings, you have a series of symbols that someone can interpret as "these chunks of concrete data represent file x". Now, if I don't need that file anymore it is a nightmare to sit there with a stick and etch out all the data I wrote. It is as taxing as writing the file to begin with. Instead, I change the symbols so instead of saying "these chunks are relating to these files" it says "There is nothing here of interest, use them if you want." You basically make the responsibility of overwriting old data to the new file being written. The new file written doesn't care because that write operation has to write regardless of whether there as data there or not. It makes no real difference whether it is all zeros or all ones or some combo, the new file has to arrange it the way it has to arrange it.