The spectrum of light is a smooth gradient with no sharp borders. Yet Isaac Newton, who split it with a prism in the 1660s, chose to name seven colours, echoing the seven notes of the musical scale. To reach that number, he added orange and indigo, a shade many people still struggle to tell apart from blue.
Cooled below −183 °C, oxygen turns into a pale blue liquid. Pour it between the poles of a strong magnet and, instead of flowing away, it hangs suspended between them. The reason lies in its unpaired electrons, which respond to the magnetic field. Liquid nitrogen, by contrast, is colourless and pours straight through.
In 2012, Felix Baumgartner jumped from a capsule hanging beneath a balloon, about 39 kilometres up. In that extremely thin air almost nothing slowed him down, and he reached around 1,358 km/h in free fall, or Mach 1.25. He became the first person to exceed the speed of sound with nothing but his own body.