This is my favorite rule. I think it's a forgotten guideline in OO design to tell objects what to do with their state rather than asking for their state. Applying this rule to code changed the way I had to write code most of all rules. Keeping classes under 50 lines changed a lot but this rule is much harder to apply than just extracting methods and classes. You really have to rethink the way you write your code I think. Actually I think that if you're going to try object calisthenics and only want to use one rule, this is probably the rule you should use.