Did you solve it? Do you have the mind of an engineer?

Earlier today I set you five ‘reverse engineering’ picture puzzles, in which I presented pictures of five structures and asked you to explain why they were built in that way. Below are the questions and the correct answers, but please do scroll below the line in the original article because some of the comedy wrong answers are very funny. Feel free to add to the tally at the bottom of this article too.

1. Baffling bridge.

bridge puzzle

The Macclesfield canal opened in 1831 and it features several ‘snake bridges’, or ‘roving bridges’, where the towpath goes from one side to the other in a loop, like the one illustrated above (and photographed here.) Why were they built like this? Think about what the canals were originally built for, and look for clues in the picture.

Answer: The bridge allows horses pulling barges to change sides of the river (when the towpath changes sides) without disconnecting the tow-rope. If a horse pulling a barge went over a normal bridge to swap sides, the rope would go over the bridge, meaning that in order to get the barge to pass through, the person in charge would have to disconnect the tow-rope, push the barge under the bridge, and then reconnect on the other side.

2. Loopy trains

train track puzzle

In some places in the world train tracks make a giant loop and then pass under themselves. Why?

Answer: In order to gain altitude while keeping a low gradient, such as in Tehachapi railway loop pictured above. Trains are heavy and cannot move up steep inclines.

3. Puzzling Polynesians

catamaran puzzle

Why did Polynesian boatbuilders make canoes that looked like the one above?

Answer: The support float, or outrigger, gives the boat more balance and lessens the chance of it being capsized in the wind, or in big waves. Indeed, the increased stability of this design means that the hulls can be narrower, longer and more hydrodynamic, and therefore much faster than other types of canoes.

4. Tricky trams


Why are the tram’s overhead cables positioned to make a zigzag, rather than straight line?

Answer: The metal structure on the roof of the tram, the pantograph, rubs against the cable as the tram moves forward. If the cable was in a straight line, it would rub the same point on the pantograph, which would begin to fray. But if the cable is in a zigzag, the rubbing happens evenly across the top of the pantograph, and the pantograph wears down less quickly.

5. Perplexing pipes


In Russian cities, most central heating is provided by pipes that have hot water or steam pumped through them from heating plants a distance away. Why do these pipes sometimes have unusual bends in them, like the one above?

Answer: The bends are there to deal with the problem of expanding metal. When hot water goes through the pipes, the metal expands and the pipes increase in length. The bends provide some slack, absorbing the extra length, and thus allowing the pipes to expand without buckling or breaking.

Thanks to Nikolai Andreev for suggesting today’s puzzles, which are taken from Kvantik, a Russian magazine about maths and physics aimed at primary school children.

I hope you enjoyed today’s puzzles. I’ll be back in two weeks.

I set a puzzle here every two weeks on a Monday. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.

I’m the author of several books of maths and puzzles, and also the co-author with Ben Lyttleton of the children’s book series Football School. The latest in the Football School series is The Greatest Ever Quiz Book, out now!

I give school talks about maths and puzzles (online and in person). If your school is interested please get in touch.


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