I have a science dedicated site, and I am using for posting articles on different subjects. Each article should be classified as 'easy', 'medium' or 'hard'.

So what I did is to create different categories (category 1, category 2, etc), and 3 'special' categories - easy, medium and hard. Each article goes to one of easy/medium/hard categories and one of the other categories (the real subject).

Problem arises when I want to display articles either by easy, medium and hard categories, or by subjects.

What I want to accomplish is two different views for the articles. One view will display 3 categories - easy, medium and hard. The other view will display all the other categories.

First view:
Easy
Article 1
Medium
Article 2
Article 3
Hard
Article 4

Second view:
Positron
Article 4
Article 2
Electron
Article 1
Radiation
Article 3

Is there a clean way to do it?

I thought of adding topics as well, IE replace subject categories with subject topics and place them under easy/medium/hard categories, but since topics cannot belong to different categories, that won't work...

So, is there a clean way to accomplish my goal?

if i can remember you can do such a thing by create "topics" in categories.
sorry i must reread documents about "topics"

You are right, it can be done, however the topics won't be shared by different categories. For example, 'easy' topic in positron category won't be the same as 'easy' topic in electron category, so this is not the solution I am trying to find.

If only topics could be shared by categories...

Each article you create can have multiple pages eg. positrons >>> 1st page (easy) 2nd page (medium) 3rd page (hard).

Not sure if that will suit you, just an idea ;o)

Edit: I just did a quick test it looks like when you make an article with multiple pages, you can add the pages to multiple categories, it was just a quick test, it may be worth a try for what you want to achieve ;o)

Oh bugger not sure an individual page within an article can be added to a separate categoory, anyway see how you go.