In water, light slows to about 75% of its speed in a vacuum. Highly energetic particles can then overtake it without breaking relativity. They emit a shock wave of light, much as a supersonic jet produces a sonic boom: this is Cherenkov radiation, the blue glow that lights up nuclear reactor pools.
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.