Was preparing to teach forces for physics in tuition, then I found this JC worksheet that describes an amazingly non-intuitive phenomenon. It is about this boy weighing 320N sitting on a chair of 160N and yet requiring only > 240N of force to pull himself up a tree via a perfect frictionless pulley system. When another person outside of the system attempts to pull the boy and his chair up by himself, he finds himself burdened with 480N just to keep the boy and his chair motionless in air. That's pretty non-intuitive eh....
Lesson no.1 Do not burden others to pull yourself up. You can do it yourself with half the effort.
Lesson no. 2 Newton is always right (for most cases)