Working out how to pass through an El Al airport security check unscathed is a conundrum that will test the most-seasoned traveller.

Spare a thought, then, for a group of students from St Paul's School in central London, who last week faced the challenge of transporting an impenetrable safe box to Israel - one that not only contained sharp metal wires inside, but could only be opened by solving cryptic physics riddles.

The group of five, who are in year 12 at the boys' school in central London, were on their way to the Weizmann Institute of Science in Tel Aviv to compete as finalists at the 21st International Physics Safe Cracking Tournament, having won the UK round.

And luckily, they made it past security, because they ended up winning the whole competition.

"We spent the past couple of months designing and constructing a safe which could only be opened by working out two physics riddles," explained pupil Benjamin Yass, 17, from north-west London, who focused on the mechanical side of the project.

Our safe could only be opened by working out two physics riddles

"Our theme was a theme park, so we had a rollercoaster inside the safe. When you complete the first physics riddle, the rollercoaster is started. The second physics riddle releases a ball bearing, which you receive as your prize.

"Our safe was based on two physics principles: that when you spin a cup of water, water goes up the sides, and, when you heat a material called SmartWire, it bends to a pre-made shape. We set it to a shape that, when heated, enableds you to reach a switch that otherwise is not accessible."

The competition, which was judged by professors and PhD students at the Weizmann Institute, gathered 30 teams, from eight countries, who had all claimed victory at their national events. Each team not only built and presented their own safes, but had to crack into each other's safes. They were then quizzed by the judging panel on physics principles.

"To crack into other people's safes, we looked for clues as to what might be the physics principles involved," Benjamin, whose family belong to Belsize Square Synagogue, explained. "For example, if we saw a coil of wire, we knew that might be something to do with electromagnetism."

Benjamin said he and his teammates, Lennie Wells, Thomas Foster, Ashwin Ahuja and Harry Armitage, had spent every lunch and break-time over the past four months perfecting their safe and brushing up on their physics knowledge. They are now diverting their attention to revising for their AS-Levels, but already have their sights set on next year's competition.

But, he stressed, their new-found safe cracking skills would only be used for good, adding that they had "no plans to carry out any bank heists or Hatton Garden break-ins".