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Can You Solve This River Crossing Puzzle?

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River crossing puzzles are a classic form of logic puzzle. In them, you're provided with a scenario—some number of entities trying to cross a river using a raft or boat—and a set of constraints (typically, some of the entities might eat each other under certain circumstances).

In the TED-Ed video below, we tackle a variant of the puzzle in which a group totaling six, three lions and three wildebeest, need to cross a river using a raft. Only two animals can go at once. The problem is, if the lions ever outnumber the wildebeest, they'll eat them. How can they all cross the river?

The larger question of this puzzle is how should we solve such puzzles? In the video, the narrator walks through this solution, but explains how it can be generalized by drawing up decision trees. At each step of the puzzle, you lay out all the possible options, then cross out any that don't work. As you proceed, the set of possibilities dwindle until you're left with only a few viable paths.

Here are the conditions for this puzzle (also listed in the video):

1. The raft needs at least one animal to paddle it across the river, and it can hold at most two animals.

2. If the lions ever outnumber the wildebeest on either side of the river (including the animals in the boat if it's on that side), they'll eat the wildebeest.

3. The animals can't just swim across, and there are no tricks; the animals have to use the boat as described.

Tune in to see how it's done:

For more on this puzzle, check out this TED-Ed page which explains its relationship to the The Missionaries and Cannibals Problem.

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Can You Guess the Secret Word in This Brain Teaser?
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On his YouTube channel Mind Your Decisions, Presh Talwalkar shares logic puzzles dealing with geometry, statistics, and algebra. The puzzle below from the former Stanford math and economics student features no numbers, but that doesn’t mean it's easy to figure out.

To solve the brain teaser, you need to guess the secret word based on a few clues. Here’s the set-up: A teacher is leading a class and Albert, Bernard, and Cheryl are his students. He writes the words "cat," "dog," "has," "max," "dim," and "tag" on the board. He distributes one sheet of paper to each of his three students, with each piece containing a different letter from one of the words. He then tells them that together their letters spell one of the words on the board. The students only know their letter, they don’t know anyone else's.

The teacher asks Albert if he knows the secret word. Albert says yes, he does know it. Next, the teacher asks Bernard. After some hesitation, he replies that yes, he knows the secret word as well. Finally, the teacher asks Cheryl if she knows what the word is. She thinks for a moment and says that yes she does. Albert, Bernard, and Cheryl have successfully guessed the secret word. Do you know what it is based on their answers?

Figuring out the word without knowing any of its letters may seem difficult, but it’s not impossible. If you don’t know where to start, think about Albert’s answer and use the process of elimination to rule out some of the letters and words written on the board. Keep in mind that Bernard could only come to his conclusion from Albert’s answer, and Cheryl from Bernard’s.

Still lost? If you haven’t gotten to the bottom of it yet, the correct answer is "dog." When Albert answers that he knows what the right word is based on one letter, you can use that information to narrow down his possible letters to one of the six that are never repeated on the board: c, o, h, s, x, and i. And when Bernard says that he knows too, you can deduce that his list of potential letters is limited to t, g, h, or s. That leaves "cat," "dog," and "has" as the three remaining options. Cheryl’s answer confirms that she has the letter d, which means the secret word is "dog."

If you’re looking for a more detailed walkthrough of the puzzle-solving process, check out the video from Presh Talwalkar below.

[h/t Mind Your Decisions]

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Can You Get to the Bottom of This Coffee Brainteaser?
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Is your brain awake and energized? If not, you may want to grab a cup of coffee to figure out this head-scratching puzzle.

According to IFL Science, the brainteaser was shared by Twitter user @_herbeautyxo and has been stumping web users ever since. The image shows coffee being poured into a network of pipes and boxes. It seems there are four places the liquid could end up and each is represented by a numbered cup. Based on the shape and arrangement of the pathways, you have to guess which vessel will catch the coffee first.

Plenty of users had guesses, but few of them answered correctly. But once you know what to look for, the puzzle becomes deceptively simple (scroll down if you want to find out the answer). 
 
 
 
 
 
 
 
 
 

Three of the four pipes are blocked off, so the only possible spot for the coffee to exit from is the remaining pipe above cup five.

Your brain doesn’t always interpret what you see in front of you accurately, even when it’s given a caffeine boost. If you need more evidence, check out these award-winning optical illusions and brain puzzles.

[h/t IFL Science]

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