In 1842, Christian Doppler predicted that a wave's frequency shifts when its source moves. In 1845, Dutch scientist Buys Ballot tested it with musicians playing a steady note on an open railway wagon. On the platform, listeners with perfect pitch heard the note rise as the train approached, then fall after it passed, just like a siren.
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.