In 1917, Einstein predicted stimulated emission: a photon can prompt an excited atom to release a twin photon, with the same color, the same direction and perfectly in step. At the time, nobody saw any use for it. It took over forty years to turn it into a machine whose very name spells out the idea: LASER stands for Light Amplification by Stimulated Emission of Radiation.
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.