[Reasoning] One

In an airtight room, which is a prison, there is only a small iron-barred window that even a child couldn't crawl through. There is someone guarding the door, and even a fly would find it hard to get in. The rusty iron door is locked tightly from the inside. Inside is a prisoner who has been held for two years, but strangely, on the day the court summons arrived, he died mysteriously, his head cut off. The iron door was locked, and there was only one key, which was with the officer. This room had no tools for committing the crime; only the small window and the lock on the door had bloodstains. Where did the head go? How did the murderer kill the person? Experts, please provide a reasonable explanation. It's best not to use Baidu.