In 1209, On A Hot Summer Day, There Was A Man Who Was Ordered To Be Executed By The King

Riddle: In 1209, on a hot summer day, there was a man who was ordered to be executed by the king. The king was a nice king though, and he said that if the man could prove himself wise, he would let him go. The king filled a room with fake flowers and only put one real flower in the room. The king said the man had 20 seconds to find the real flower. The man said, "It's very stuffy in here. Could you open a window?" The king opened a window, and the man immediately knew which was the real flower. How did he know?