Since an international agreement in 1959, the inch has been exactly 2.54 centimetres and the pound exactly 0.45359237 kilograms. Imperial units no longer have standards of their own: they are all fixed multiples of the metric system. Even countries that measure in feet and miles rely, behind the scenes, on the SI metre and kilogram.
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.