In air, the speed of sound depends mainly on temperature: about 331 metres per second at 0 °C, plus roughly 0.6 m/s per degree. The warmer the air, the faster its molecules jostle and pass the vibration on. That is also why an aircraft's "Mach 1" is not a fixed speed: it drops at altitude, where the air is freezing.
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.