On the site I'm building, users can create tutorials. I want the users to be able to create tutorials on as many subjects as possible, but still have some preset categories. What's the best way to select these categories?

The reason I don't just let users add keywords, and use these for categorization, is because users gain experience points in a certain subject when their tutorial is liked by someone, and in a similar way the Stack Exchange network does, create communities around these subjects. I will give visiters the possibility to suggest new categories.

here are the categories that I'm thinking of at the moment:

• health
• gardening
• cooking
• technology
• science & math
• music
• visual art