A crown is Martina König's favourite piece - not a princely one, but one worn by revue dancers. "On my tours with children, the crown of the revue dancers is the highlight," says Martina König. Because the opulent piece of jewellery not only glitters in all the colours of the rainbow, but is also perfect for explaining the physical properties of light: Why does light refract in cut glass? "Everyone is allowed to try on the crown - by the way, it is equally popular with boys and girls," reports the glass expert.