Logic riddles

You’re in a house, and there is lamp upstairs that is off. In the basement are three switches. One of the switches turns on the lamp, the other two do nothing. From the basement there is no way to observe whether the lamp upstairs is on or off. You are allowed to flip two switches, after which you must be able to say with certainty which of the three switches turns the lamp on/off. What do you do?

Answer (select grey block with your mouse to view):

You flip the first switch on, wait 10 minutes, then flip it off again. Then you flip the second switch on and go upstairs. If the lamp is on the correct answer is the second switch. If the bulb is warm the correct answer is the first switch. Otherwise it’s the third switch.

What logic riddles do you know?

PS. To hide the answer use [noparse]Answer[/noparse]

I like it :smiley:

I thought he shot helicopters down with police cars.

The first riddle is misleading because I didn’t figure out you could actually go upstairs after flipping the switches.

This one is one of my mum’s favourites, which I sometimes say to my students and they look at me blankly, even after explaining it:

  • Have you ever seen an elephant hiding behind a flower?
    - Er… no.
  • Doesn’t it hide well?

Pour 3 litres into the 5 litre, then pour 2 litres from the 3 litre into the 5 litre so that its full. There will be 1 litre left. Empty the 5 litre, pour the 1 litre in, then pour a full 3 litres in.

I was chuffed to have figured it out before Bruce Willis. I mean… the guy shoots down helecoptors with PLANES.

Oh…wait… you mean it wasn’t real? :eek:

Here’s another one (those of you who’ve seen Die Hard will know the answer).

You have two bottles. One can hold 3 gallons of water, the other one can hold 5 gallons. You have no scales, or other ways of measuring, and you need to get exactly 4 gallons of water in the 5 gallon bottle. You have an unlimited supply of water. What do you do?

  • Fill 3 gallon bottle
  • Pour water of 3 gallon bottle in the 5 gallon bottle
  • Fill 3 gallon bottle
  • Pour water of 3 gallon bottle in 5 gallon bottle until it’s full. There is now one gallon left in the 3 gallon bottle
  • Empty the 5 gallon bottle
  • Pour the 1 gallon from the 3 gallon bottle in the 5 gallon bottle
  • Fill 3 gallon bottle
  • Pour water of 3 gallon bottle in the 5 gallon bottle, which now contains 4 gallons of water

I didn’t too. Thought about it for several hours before someone gave me the answer :slight_smile:

[FONT=“Georgia”]Yeah, I saw the answer. I just didn’t figure it out.


Select the grey area with your mouse to read the answer. It’s grey text on a grey background, which becomes visible when you select it :slight_smile:

Another one from the same Die Hard movie:

As I was going to St Ives
I met a man with seven wives
Each wife had seven sacks
Each sack had seven cats
Each cat had seven kits
Kits, cats, sacks, wives
How many were going to St Ives?

1, only the narrator is known to be heading to St Ives, fore the rest where they set out from or where they are heading to is unkown

[FONT=“Georgia”]It’s on the North Pole; My Physics teacher asked us that one once. Actually in his version he said, “A big bear walks up. What colour is the bear?”

I didn’t figure out the first one.


An easy one:

In a house, all the windows face south. The house is square and all the walls have windows. How can this be?

It’s on the North Pole