Numbers have to be even, and thus a number of objects has to be divisible into at least two groups. Things have to be clean, no matter what it is. Certain things (books, pencils, anything straight) have to be arranged in a specific order, depending on the time, place and object. Food and plastic can't be touching if that's not how they normally come. Food that's touching plastic can't go in the fridge.
That's just a few.
That's just a few.