My mind was kept sharp by our four kids constant 'negotiations'. The oldest one was so good at arguing his point, but always respectfully, we told him he ought to be a lawyer. That's what he did, and as we thought he would be, is pretty good at it!
We had interesting discussions with the kids as they were growing up; still do, when they're home from college, or just visiting, so they're pretty sharp, and know how to take apart an argument when it comes to their friends trying to convince them of the latest popular notions.