I took the guns, but I didn't really understand why, or who we were waiting for. So I was pleased when Dupin began to explain his thoughts to me.

"Well," I replied, "all the witnesses agreed that the deep voice was that of a Frenchman. But none of them agreed about the other voice, the shrill one -- they all thought something different."

"Now, let's think," he said, "about those angry voices heard by the neighbours running up the stairs. Think about the newspaper reports of what the witnesses said. Do you remember anything peculiar in what they said?"

"Yes, that was what they said, but you haven't understood what's peculiar about it," said Dupin. "The peculiar thing is -- not that they disagreed -- but that they all thought it was the voice of a foreigner. Each witness thought the voice spoke a language that they didn't know. Look at this list." He showed me a piece of paper.

· One Frenchman thought the voice spoke in Spanish, but he didn't hear any words.

· A second Frenchman thought the voice spoke in Italian, because of the sound of the words, but he didn't know Italian himself.

· A Dutchman thought the voice spoke in French, but he himself didn't speak French.

· A Spaniard thought the voice spoke in English -- he couldn't speak English but he knew what it sounded like.

"That is always possible," Dupin agreed, "but do you begin to see what question we must ask next?"

· And last, an Italian thought the voice spoke in Russian, but this man has never spoken to a Russian person.

"But perhaps the voice was speaking an African language," I said. "Or an Asian one."

· An Englishman thought the voice spoke in German, but he didn't understand German.

"So! What a strangely unusual voice this was!" said Dupin. "Speakers of five European languages did not hear one word -- not one word -- that they knew. It was also a strangely shrill voice, and was it a man's voice or a woman's? No one could tell."

Puzzled, I shook my head.

"Well, we will come back to the voice later," said Dupin. "But for me, my thoughts about the voice already told me what to ask next. So, let us close our eyes and remember that room on the fourth floor of the house in the Rue Morgue. What do we want to find out first? The way the murderer got out of the room. Let's think about all the possible ways. First, the door to the passage was locked, with the key on the inside. We cannot argue with a key in a locked door. There were no secret doors -- the police have looked at every centimetre of the floor, the ceiling, and the walls. And I also looked very carefully. So, no secret doors. What about the chimney? It is wide enough for a body for three metres, but higher up it is much narrower. Not even a cat could climb through it to the top. So what is left?"

"But I went on thinking. The windows were the only possible way, so the murderer did escape from one of these windows. But they were fastened on the inside, and how did the murderer fasten the window again -- from the outside? There was only one answer. The window could fasten itself. I took out the nail from the window on the left, but the window still would not open. So I was sure there was a hidden spring somewhere, and after a while I found it. I pressed it, arid then I could open the window."

"The two windows," I said. "But they were fastened on the inside, weren't they?"

"Yes, and no," said Dupin. "Let me explain. We can see all of the window on the left, you remember, but only the top half of the window on the right, because the head of the bed is pushed up next to the window. The police tried to open the window on the left, but found a very strong nail in the wood, which stopped the window opening. Another big nail was found in the other window. And the police stopped there. No one could get out of these windows, they thought, because of the nails and because both windows were fastened on the inside. So they did not try to take out the nails and open the windows."

"Ah," said Dupin, "that was the next question. You remember, don't you, how we walked around the building? And you saw, didn't you, the lightning-rod that went up the back wall of the building? And the unusual shutters on the windows on the fourth floor?"

"So," I said, "we know it was possible to escape throught the window behind the bed head. But the room was on the fourth floor…"

"I put the nail back into its hole, and thought some more. The murderer gets out through the window, then the window drops down again behind him, and fastens itself by its hidden spring. But the nail -- how could anybody put back the nail from the outside? I went to the other window and stood on the bed, looking down behind the bed head at the bottom half of the window. The nail in this window looked the same, but… There must be something wrong with this nail, I said to myself. I touched it -- and the head of the nail came off in my fingers. The rest of the nail stayed in the wood. I carefully put back the broken nail head, pressed the hidden spring, and lifted the window a few centimetres. The nail head went up and came down with the window, but still looked like a real nail."

"Yes," Dupin said, "and very easy for a hand to get hold of. Now, let me describe a possible picture to you. The lightning-rod on the wall is less than two metres from the window by the head of the bed. The latticed shutter is more than a metre wide, and when it is open and against the wall, it is only about half a metre away from the lightning-rod. It is possible to climb up the lightning-rod to the fourth floor. Then, a strong and agile -- very agile -- person could take hold of the latticed shutter with both hands, push his feet against the wall, and swing himself and the shutter across the window. And if the window is open and this person is very agile indeed, he could swing himself into the room."

Dupin saw the surprise in my face. "Remember," he said, "that I am talking about somebody who is very strong and agile -- agile in a very unusual way, perhaps. Remember also the voice, that peculiar, shrill voice, which spoke in a language that nobody knew."

"Yes," I said, "they were like doors, but the top half was latticed, which is unusual in Paris."

At these words I felt I almost understood what Dupin was saying. But I wasn't sure, so I said nothing and waited for him to go on explaining.