Horsepower was popularised by James Watt in the late 18th century to sell his engines against draught horses. It equals about 745 watts, estimated from a horse's steady work over time. But at its peak, a horse can produce around 15 horsepower, according to a study published in Nature in 1993. The unit describes endurance, not maximum effort.
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.