In the late 18th century, scientists wanted a unit taken from the Earth itself, so it could belong to everyone. The metre was set at one ten-millionth of the distance from the North Pole to the equator, surveyed by triangulation over several years. A small error crept in: the standard metre came out about 0.2 millimetres too short. It was kept anyway.
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.