What's your favorite lateral thinking puzzles?

I started a new thing this term where my tests include a lateral thinking puzzle as extra credit. It’s been a huge hit so far, but I’m having trouble finding new ones.

Criteria:

1. The puzzle is short enough to write on the board.
2. The answer is short enough that it an be written in maybe 2"-4" of extra space on a regular a4 paper (leftover room or border on the test.)
3. The answer is relatively simple and triggers an “aha!” moment.
4. The rules are not too complicated for low-level language learners.
5. Minimal math.

Ones I’ve used recently:

1. The old lettuce/sheep/wolf/boat crossing the river riddle.
2. Matchstick puzzles: today’s was The Donkey
3. What’s next in this sequence: j, f, m, a, m, j, j, a, s, o, n, ____?

There’s a few that might be suitable in this “chestnuts” thread at the Grey Labyrinth puzzle site: http://www.greylabyrinth.com/discussion/viewtopic.php?t=12946

e.g. the basic “Knight & Knave” puzzle:

As you’re trying to find your way out of a forest you reach a fork in the path. You know that one of these paths will lead you out of the forest and the other will send you to your doom.
At this fork stand two men: One is a knight who is honorbound to tell the truth in all cases; the other is a knave who will lie with every statement. However, you do not know which man is which.
What one question can you ask to one of the men in order to find your way out of the woods?

 actually, that’s a bit long to write on a board. How about:

*You are at the top of the stairs and you see three light switches. You know that they control the three bulbs at the bottom of the stairs (around a corner so you can’t see them from here). You would like to know which switch controls which bulb. What is the minimum number of trips down the stairs to glean this information? *

Made a lateral move from Cafe Society to The Game Room.

As an alternative to the sheep/lettuce/whatever puzzle, try this: Three married couples must cross the river in a boat big enough for only two of them at a time. Because of tribal taboos, no wife may ever be left with another man in the absence of her husband - even if the other man’s wife is also present. Any one man or woman can manage the boat.

I’m guessing you’ve already seen and used the books on the subject including the “Whack on the side of the head” ones?

I have (or had) five or six “lateral thinking” puzzles books around here somewhere, if you haven’t seen them. I imagine (haven’t looked yet) they’re still available at Amazon and similar places.

Three switches control three light bulbs in another room. You can play with the switches all you want, but once you enter the room with the bulbs, you need to identify which switch controls which light. How can you do this?

Turn on switch 1 and 2. After ten minutes, turn off switch 2. Enter the other room. The light that is on goes with switch 1. Touch the remaining light bulbs. The hot one goes with switch 2, the remaining has to be switch 3.

In TommyTutone’s formulation of the lightbulb problem, it is vital that you know which switch position is on and which is off before entering the room. Ximenean’s formulation can be used either way, but with different answers. (That having been said, I imagine they were going for the same thing). Make sure you make such details clear and unambiguous whenever necessary.

(Unfortunately, another point of ambiguity is whether one can only obtain information by looking at the lights, or one is allowed to touch them as well. But clarifying this ruins the “punchline”…)

Imagine a square pond, 50 feet to a side. At each corner of the pond there’s a beautiful oak tree planted. It’s one of those strange oak trees whose roots go straight down and who otherwise completely don’t act like normal trees but exist only to mark the points of the pond in the puzzle, so don’t go getting all botanical on me.

Anyway. The owner of the pond loves the four oak trees. She loves having a square pond. But it’s not big enough. She wants to expand it so that it’s twice as big as it is now (and before you get all clever, all she cares about is surface area, not depth).

How can she do it? How can she leave those four oak trees where they are, maintain a square pond, and double its size?

In answer to LHoD’s puzzle, Rotate the pond 45 degrees and then enlarge it, so the trees become the midpoints of its edges, rather than the corners

Very nice, I was trying to work out how to dig little islands around the trees, and enlarge the pond to 5000ft[sup]2[/sup], plus the surface area of the islands.

Check out the movie Fermat’s Room; much of the action revolves around a bunch of these puzzles. Subtitled, but a decent enough watch nonetheless.

I’m having trouble remembering my favorite puzzle from the movie. It was something like three types of tea in three boxes: two binary choices and the third an equal combination of the two. Sample two and then tell which box holds which. Something like that. Or maybe it was sample one and then determine which of the three holds a given type. I can’t remember.

Based on the IMDb threads, here are some more from the movie:

*The teacher has 3 children. One of his students asks their ages. The teacher’s replay:

If you multiply their ages together, the result is 36.
If you add their ages together you get your [the student’s] house number.

The student replies that he needs more information before he can determine the answer. The teacher then replies:

Oh yes! The oldest one plays the piano.*[spoiler]The key to this is how many combinations of 3 numbers which when multiplied together will total 36?

36 = 6 X 3 X 2 ; Sum = 11
36 = 4 X 3 X 3 ; Sum = 10
36 = 6 X 6 X 1 ; Sum = 13
36 = 9 X 2 X 2 ; Sum = 13
36 = 9 X 4 X 1 ; Sum = 14
36 = 12 X 3 X 1 ; Sum = 16
36 = 18 X 2 X 1 ; Sum = 21

There are 2 combinations which have the same sum. Since the student claims he needs more information, then one of those two must be the answer. When the teacher says that “the oldest child plays the piano,” we now have our answer. The only combination where there is one oldest child is the one with 9, 2, and 2. Oh and the student’s house number is 13.[/spoiler]

A mother is 21 years older than her son. In 6 years, the son will be one-fifth his mother’s age. What is the father currently doing?Impregnating the mother.

It also included several already mentioned in this thread: animals crossing a river in a canoe, truthteller and liar, and switches with lightbulbs in another room.

Googling around trying to find the tea one I mentioned I ran across another one.

There are six eggs in the basket. Six people each take one egg, how can it be that one egg is left in the basket?

I don’t see the answer online, but my answer is:The last person takes the basket with the last egg in it.

Don’t forget the Monty Hall problem. Easy to write both the question and answer, but hours of endless debate:

You’re given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what’s behind the doors, opens another door, say No. 3, which has a goat. He then says to you, “Do you want to pick door No. 2?” What should you do and why?Switching to door #2 doubles your chances for the car, since door #1 only had a 1/3 chance to be correct. Switching gives you a 2/3 chance for the car.

A lot of these puzzles are more straight-up math puzzles than the kinds of lateral thinking puzzles the OP wants, I think. But that eggs and basket one is along the desired lines, I think. And, of course, everyone likes the punchline to “What’s the father currently doing?”.

Agreed that they’re more math puzzles than traditional lateral thinking puzzles, but that is in keeping with the precedent the OP set with his example question #1.

Found the tea question proper, though this version uses fruit instead:

*You have three boxes. One is labeled “Apples”, one is labeled “Oranges” and one is labeled “Apples and Oranges”. All three are labeled incorrectly. You can pick one fruit from one box. How will you relabel the boxes correctly? *The key is that they’re all labelled wrong. Pick a fruit from the “Apples and Oranges” box; it must be all of whatever you get. So put that label on it and switch the other two labels.

Yes, when I said “A lot of these puzzles”, I didn’t just mean yours, but those throughout the thread, including the OP’s #1, as you note.

A bear walks ten miles south, then ten miles east, then ten miles north, and ends up right where it started.
.
.
.
What color is the bear?

White.

Four philosophers - Plato, Socrates, Diogenes and Aristotle - had thirteen small cakes between them on a plate and they helped themselves as they went along, too busy to pay very close attention as to whether each man received his fair share. After a while, all the cakes were eaten, and the following discussion ensued:

Plato: Did you eat more cakes than I did, Socrates?
Socrates: I don’t know. Did you eat more cakes than I did, Diogenes?
Diogenes: I don’t know.
Aristotle: Aha!

Aristotle now knew how many cakes everyone had eaten - and, of course, knowing that Aristotle knew, the other three were able to make the same deduction. Given that:

• All four were extremely intelligent, flawlessly logical, and knew that their fellows were too
• Prior to the discussion, each of them knew only that everyone had eaten at least one cake
• Each knew how many cakes he himself had eaten
• None of them would dream of asking a question to which he already knew the answer

How many cakes did everyone eat?

[spoiler]Socrates knows that Plato ate at least one cake. His reply tells us that he himself ate at least two, or else he would have answered “No”.

Diogenes, knowing that Socrates ate at least two cakes, does not reply “No”, which would have meant that he himself ate only one or two, so he must have eaten at least three.

Plato now knows that the first three philosophers ate at least six cakes between them. The fact that he also knows exactly how many cakes everyone ate proves that he must have eaten seven, as if he had eaten fewer, any of the other three could have eaten more than the minimum.

Therefore Plato ate seven cakes, Diogenes ate three, Socrates ate two, and Plato ate one.[/spoiler]

Aristotle: Hey, you ate my cakes!

Oops. The correction is left as an exercise for the student. :o :smack:

I know that this answer is based on the bear being at the North Pole, but there is actually another region on Earth that fits the requirements of the puzzle. Anyone know it?