Big-bang. A tiny fraction of a second after the Big Bang (first two shells) the temperature is so high that the universe is formed only by a mixture of exotic particles (coloured spheres) and radiation (squiggles). After the first three minutes (fourth shell) the temperature has dropped to 1 billion Kelvin and protons and neutrons (large spheres) are formed. 300,000 years later (fifth shell) matter and radiation do not interact anymore and atoms start to form. In the seventh shell, 500 million years after the Big Bang, stars are born and heavy elements, the progenitors of life, are formed in them as a by- product of nuclear fusion reactions.