The bridge will collapse in 17 minutes! Four people need to cross the bridge before it collapses. It is a dark night and they have only one flashlight among them.

Only two people can cross at a time.

Alice takes one minute to cross.

Bob takes two minutes.

Carol takes five minutes

Dave takes 10 minutes to cross.

How can they all get across before the bridge collapses? Click below to see the answer.

The trick to this puzzle is to get the slowest members of the group to cross only once together, while the fastest members cross back and forth multiple times.

Alice and Bob cross first using up 2 minutes.

Alice comes back making it 3.

Carol and Dave cross together making it 13 minutes.

Then Bob crosses back, making it 15 minutes.

Finally, Alice and Bob cross together to make it a total of 17 minutes.

Raymond Smullyan, one of the grand masters of logic puzzles, sadly passed away at the age of 97 earlier this week. In his honor, I present a classic puzzle adapted from his book What Is the Name of This Book?

There is a wide variety of puzzles about an island in which certain inhabitants called "knights" always tell the truth and others called "knaves" always lie. It is assumed that every inhabitant of the island is either a knight or a knave.

In this problem, there are only two people, A and B, each of whom is either a knight or a knave. A makes the following statement: "At least one of us is a knave." What are A and B?

Click below for the solution.

The solutions to these puzzles are often found by making one or more assumptions, then reasoning out whether or not it can be true. In this case, assume A is a knave. Then the statement "At least one of us is a knave" would be false, since knaves always lie. Hence, both A and B would be knights, which is impossible because we started with the assumption that A is a knave. Therefore, A must be a knight, and the statement "At least one of us is a knave" must be true, and B is a knave.

Raymond Smullyan presented a couple more of his puzzles in a 1982 interview on the Tonight Show with Johnny Carson. When you see the white hair and long beard, it seems like even 35 years ago that Smullyan was an old man, but the twinkle in his eye and the playfulness in his voice reveal that he was always a child at heart. Watch the full interview below.

If these puzzles seem too easy, they're just a small sample of Dr. Smullyan's brilliant work. If you really want a challenge, I encourage you to check out some of his books, or The Hardest Logic Puzzle Ever, also credited to Smullyan.

Finally, I leave you with a quote.

Why should I be worried about dying?
It's not going to happen in my lifetime!

The guinea pig is not a pig. It is a rodent. (And it's not even from Guinea, a country on the west coast of Africa. Guinea pigs originated in the Andes mountains in South America.)

When this book first came out, it was only read by a handful of rather wealthy people. Now almost everyone has a copy, and many people read it regularly. You cannot buy it from a bookstore or borrow it from the library. Can you name this book? Click below for the answer.

It's a phone book.

This is a good example of an old riddle that has not aged well, so don't feel bad if you found it a bit tricky. Newer technology has rendered phone books practically useless, yet I still get one delivered to my house every year. In another generation or so, many people might not know what they were used for.

At the outbreak of World War I, none of the combatant nations provided steel helmets to their troops. Soldiers of most nations went into battle wearing cloth or leather hats that offered little protection from modern weapons. As a result, many soldiers suffered head injuries from exploding shrapnel.

In April of 1916, British soldiers began using a metal helmet in battle called the Brodie helmet, but authorities discovered that the proportion of head injuries then increased. Why should the incidence of head injuries increase when soldiers wore metal helmets rather than cloth caps? Click below to see the answer.

The number of recorded head injuries did increase after the introduction of the Brodie helmet, but the number of deaths decreased. Prior to the introduction of metal helmets, if a soldier were hit in the head with a piece of shrapnel, it would have likely killed him. This would have been recorded as a death, not a head injury. More head injuries were recorded after the helmets were introduced due to the simple fact that more soldiers were surviving them.

A ship sailed through the Panama Canal going from west to east. When it exited the canal, it entered the Pacific Ocean. (The ship did not double back.) How can this be so? Click below to see the answer.

Normally when we think about the American continents, we think of the Pacific Ocean being to the west and the Atlantic Ocean to the east, but that isn't strictly the case. Panama is an isthmus that curves, and the Panama Canal was constructed so that it runs from the Caribbean Sea on the northwest end to the Pacific Ocean on the southeast. In the age of instant access to world maps, this "puzzle" is probably a lot easier to verify now than when the canal was originally built.

Suppose you tie a rope tightly around the Earth at the equator. (Assume the Earth is perfectly spherical, and that the surface is smooth so that the rope lies tight against the surface at all points.) Now suppose that you add an additional 6 feet to the length of the rope. How high off the surface would the rope lie? You could look up the Earth's circumference and do the math to come up with an exact answer, but can you quickly come up with an intuitive guess? (High enough to slide a piece of paper under? To wave your hand under? To walk under?) Click below to see a hint or the answer.

Hint: If I reversed the parameters and told you that I increased the length enough to raise the rope 6 feet from the surface in all directions, could you tell me how much was added to the length of the rope? (Given the formula for the circumference of a circle, C = 2πr, but not knowing the circumference of the Earth, can you come up with a guess?) Reversing that, can you come up with the answer to the original problem?

Answer: The first time they hear this puzzle, many people will try to do the math starting with the circumference of the Earth. That doesn't matter though. It's a property of any circle that if you increase the circumference by a fixed amount, the radius will change by that amount divided by 2π (because r = C/2π). The rope could be tied around a beach ball or a tennis ball and the answer would not change. So the exact answer to the problem is 0.95493 feet, but if you said "about 1 foot" you were right.