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Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientiﬁc enquiries about everyday phenomena. The writers of answers published in the magazine will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format. New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.newscientist.com/topic/lastword (please include a postal address in order to receive payment for answers). To view unanswered questions visit www.newscientist.com/topic/lastword. THE LAST WORD Candle in the wind My birthday cake had candles on it that I couldn’t blow out. How do these work? n The short answer is that they have magnesium incorporated into the wick. For a flame to exist, it needs oxygen, fuel and enough heat to keep combustion going once it starts. These requirements are sometimes represented graphically as the three sides of a “fire triangle”. When you blow out an ordinary candle, you extinguish the flame by removing heat. You can still see the fuel – the wax smoke, often paraffin vapour, coming from the wick – but the ember in the wick does not supply enough heat to reignite it, and so it will eventually go out. But when the wick has magnesium powder in it, as was the case with the candles on your correspondent’s birthday cake, the ember is able to ignite the magnesium. This element catches fire at relatively low temperatures, below 500 °C – and this is enough to reignite the smoky fuel. The flame itself then burns at around 3000 °C. If you look closely at one of these candles after you have apparently blown it out, you can see the smouldering wick emitting little sparks of burning magnesium before the candle relights. Oh, and a belated happy birthday to your correspondent. David Muir Science department, Portobello High School Edinburgh, UK Airscrews Any idea how these clouds formed above San Francisco Bay (see photo)? They look like a corkscrew. n The clouds are a manifestation of Kelvin-Helmholtz waves. Despite their appearance, they are not breaking waves like those you see on the seashore. Kelvin-Helmholtz waves are produced when there is wind shear between layers of air, such as when two layers, one above the other, are moving at different velocities. The waves cause humid air to rise, cooling it below the dew point and allowing cloud droplets to form. Such waves in clouds are extremely short-lived, usually dissipating within a few minutes – and so often go unseen. I have seen them twice. On one of those occasions, just the wave crests were visible for less than a minute. Kelvin-Helmholtz waves may also form when there is a difference in velocity across the boundary between two different fluids. They are observed in the ocean, and frequently occur at the boundaries between atmospheric belts on Jupiter and Saturn. Storm Dunlop Chichester, West Sussex, UK This week’s questions LONG-HAUL RACERS Can any land-based animal outperform an elite human marathon runner over the full distance? If so, which ones? Michael McCullough London, UK MAGNETIC MARBLES, ANYONE? Is it possible to build a spherical magnet? If we could manage to do it, where would the north and south poles be, and what could such a magnet be used for? V. Taylor Milton Keynes, Buckinghamshire, UK SOFT FOCUS Here’s something for all rich tea fans who like to dunk the biscuit in their tea. Try dunking it first in hot, black, unsugared tea. When you bite into the dunked portion, you will notice that although wet on the outside, it is just as hard on the inside as if you had never dunked it. Now add milk to the tea, stir and dunk another biscuit into it. Amazingly, the dunked portion is moist and soft right through. So why the difference? Jim Watt Peterborough, Cambridgeshire, UK Rich teas are popular cookies in the UK and Ireland. They are made from wheat flour, sugar, vegetable oil and malt extract, and their consistency makes them ideal for dipping into hot tea or coffee before eating – Ed “If you look closely at the candle, you will see the wick emitting sparks of burning magnesium” Last words past and present at newscientist.com/topic/lastword The new book out now: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphins Will we ever speak dolphin?