I did several tests on the above. I have experienced the following: 1) https://codegym.cc/quests/lectures/questcore.level06.lecture08 It seems the unofficial version always worked. It stopped the child thread in all cases. Is this really true in all cases? Or was it just by coincidence? 2) https://codegym.cc/quests/lectures/questcore.level06.lecture09 The official version sometimes worked, sometimes it didn’t. It sometimes stopped the child thread, and not in other cases. Why is this so unreliable? 3) The CG lecture says: "Nobody can guarantee that a thread will be stopped." But what I have to do if I still want to be 100% sure to stop the thread?