I was watching a Walter Lewin lecture on Newton's laws (which was great, by the way), and at the end, he presented this problem; There's a string hanging from the ceiling (actually not the ceiling, it was just a surface), and a cube is hanging from that string. There is another string, attached below the cube. Both strings are massless, and the cube's mass is 2 kg. If the bottom string is pulled until one of the two strings break, which string will break, the bottom or the top one? The students in the lecture all guess that the top string will break, but when Professor Lewin pulls the bottom string, that's the one that breaks. He does it again, but this time the top string breaks! The third time, the bottom one breaks. He tells his students to think about what's going on, and the lecture ends there. I'm not really sure, but the best I can figure is this; For the top string, the tension is equal to the weight of the cube plus the tension of the bottom string. For the bottom string, the tension is equal to the applied force (because of Newton's 3rd Law: the string pulls the cube down, which pulls the string up) plus (maybe, I'm not really sure about this one) the tension of the top string. Since the forces on both strings are equal, they should snap at the same time (maybe, I'm not sure that's possible), but since there are small imperfections, one string will snap a little bit earlier. If this is right, please let me know so I can sleep tonight. If not, please tell me what's actually going on so I can sleep tonight. Either way, thanks for helping me solve this.