My cousin posed this classic riddle to me and I can't quite figure it out. He says there are 3 switches outside a closed room with a lightbulb inside, but I can only enter once. What's the logical solution?
Something similar happened to me not long ago, someone threw that at me and I didn't get it the first time. The thing is, most people think you can only use the switches when you go into the room, but the key is doing it before. What you gotta do is turn on the first switch and leave it like that for a couple of minutes, then you turn it off. After that you turn on the second switch and leave it on, and you leave the third one off the whole time.
When you go into the room, the light bulb will give you three clues: if it's on, it's the second switch (because you left it on). If it's off but hot, it's the first one (because you turned it on earlier and it heated up). And if it's off and cold, it's the third one. People fail because they assume you only have one moment to do things, but really you can mess with the switches as long as you need before you go in.
The key is that you can manipulate the switches *before* you enter the room, not just when you're inside. Most people get stuck thinking they have to go in, test the switches, and that's it - but actually, the time you spend outside is part of the solution.
Here's what you do: turn on the first switch and leave it that way for a few minutes (5-10 minutes works). Then turn it off. Leave the second switch on. Don't touch the third one. Now you enter the room just once and observe three things: if the bulb is on (that's switch 2), if it's off but warm to the touch (that's switch 1, because it was on for a while), or if it's off and cold (that's switch 3). That way you figure out which is which without needing to go in multiple times.
The trick is that an incandescent bulb holds onto heat for a while after you turn it off, so you can use that as extra information. It's a classic puzzle precisely because it plays on that wrong assumption that you can only act inside the room.
Exactly, what the other comments don't mention is that you can use *temperature* as your third variable 🔦 You turn on one switch and leave it on for a few minutes, then turn it off. Then you turn on another one and go into the room: if the bulb is on, it's the second switch; if it's off but hot, it's the first one; if it's off and cold, it's the third. It's pretty clever because most people just think about light and darkness and never consider that they can touch the bulb to detect heat.
The trick works because the lightbulb generates heat in addition to light, so by touching the bulb you can figure out which of the three switches controls it. You turn on the first switch and wait a few minutes, turn it off, then turn on the second switch and go into the room: if the bulb is on, it's the second switch; if it's off but warm, it was the first one; if it's cold and off, it's the third.
The others already explained it well, but there's a detail they left out: if you want to be completely sure which switch is which, you need to do this in order. Turn on the first one and leave it that way for 5 or 10 minutes, then turn it off. Immediately turn on the second one and go into the room. When you enter, if the bulb is *on*, it's the second switch. If it's *off but warm* to the touch, it's the first one. If it's off and cold, it's the third. People sometimes get confused because they think they have to try all three before entering, but the reality is that by only manipulating two and using temperature as extra info, you already have complete certainty about the third one without even touching it.
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