A young, healthy ear picks up roughly 20 to 20,000 hertz. With age, the hair cells that detect high frequencies wear out first, and the upper limit drops. Ringtones set around 17,000 hertz have circulated among teenagers, inaudible to most of the adults sitting right next to them.
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.