Strong knots, weak knots... If you have much experience with ropes and knots, then you already know that knots are weaker than the ropes they're tied in. What you may not know is that various kinds of knots vary in how much they weaken the rope they're tied in. When I was in the Navy, I had to complete a course in “marlinspike seamanship” – basically, the art of tying knots. We learned by rote which knots were strong (that is, weakened the rope less) and which were weak (weakened the rope more), but I had no idea why. The problem has been studied, and the reasons identified.
By the way ... the single most important thing I learned from that marlinspike seamanship course was this: figure-eight knots are good.