Today’s puzzles test your engineering smarts.

The cranberries

In a cranberry sorting facility, thousands of cranberries are dropped every minute onto a board. Good cranberries are firm, and bounce, while bad cranberries are soft, and don’t bounce.

What simple trick allows sorters to catch all of the good ones and none of the bad?

Caps off

In the US, medicine bottle caps are designed to be secure against only 85 per cent of children under five.

Why is this percentage not higher? Eighty-five seems quite low.

Slats entertainment

In 2020, engineers replaced the pedestrian handrail on the Golden Gate Bridge. The old handrail rested on metal balusters in the shape of an H, so the support looked like “HHHH…” where the H’s horizontal section was 10cm wide. The new handrail was supported only by thin vertical slats, making it look like “IIII…” and where each slat was only 0.6cm wide.

The aim of the new handrail was to make it less resistant to wind, and hence safer during high winds. But it brought two serious unintended consequences.

What were they?

Under pressure

Cabin air pressure in a plane is controlled so that it is the same as air pressure at ground level. When a plane flies, the air pressure outside the plane drops, causing the air pressure in the cabin to push outwards against the frame with great force

In the early 1950s, there were some major plane crashes when the fuselage was ripped apart mid flight.

UK manufacturer de Havilland found a way to pressure test its plane, the Comet, the world’s first commercial jet, without the plane leaving the ground.

What did they do?

I’ll be back at 5pm UK with the solutions.

PLEASE NO SPOILERS. Instead talk about your favourite engineering anecdotes.

Today’s puzzles are adapted from a hugely entertaining new book about the world of test engineering, How to Keep a Plane in the Sky, by technology reporter Alex Davies. It tells fascinating stories about the scientists whose work underpins our trust that objects do not crack, break or explode. Thanks to Alex for the puzzles. You can buy his book at the Guardian Bookshop and other retailers.

And finally…some professional news. Last week I won the 2026 Christopher Zeeman Medal, the UK’s most prestigious honour for mathematics communication, which is given by the London Mathematical Society and the Instititue for Mathematics and Its Applications. I’m absolutely thrilled and hope to continue setting mathsy puzzles here long into the future!

I’ve been setting a puzzle here on alternate Mondays since 2015. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.