I've just got back from a fireworks display where some of the fireworks exploded into a square shape. How do they do that?
Justin Muller, Tokyo, Japan
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The firework was probably an aerial shell fired from a mortar tube. Such shells are usually spherical and contain stars, or pellets of coloured pyrotechnic material, arranged around an explosive charge. This charge both ignites and propels the stars in all directions at the apogee of the shell's flight.
The stars are arranged in a square when the shell is being constructed, positioning them at slightly different distances from the centre. The propulsive force on each star will be the vector sum of all the explosive forces surrounding it; because each star is in a slightly different position, the force on each is also slightly different. Those placed nearer the centre of the shell will experience less outward force than those further from the centre - the furthest being the stars that form the corners of the square. If the stars are uniformly arranged against the shell wall they will form a spherical burst. Shapes such as hearts and smilies are made by arranging stars asymmetrically.
One potential problem is that shells tend to spin on their trajectory, so there is a possibility that spectators will see this kind of firework burst end-on. If this were to happen, the stars would appear as a line rather than a two-dimensional shape.
Tony Charity, Lowestoft, Suffolk, UK