Reusable hand warmers contain dissolved sodium acetate that stays liquid even though it should crystallise. Flexing the small metal disc creates a seed crystal: crystallisation sweeps through in seconds and releases heat, at around 50 °C. To reset it, just drop it in boiling water, which dissolves the crystals again.
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.