Since 1983, the metre has been the distance light travels in a vacuum in 1/299,792,458 of a second. A surprising consequence: the speed of light is exactly 299,792,458 metres per second by definition, and can never be measured more precisely. Before that, the metre was defined by a metal bar, then by light emitted from krypton atoms.
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.