An intact onion does not sting. Slicing it breaks its cells and brings sulphur compounds into contact with enzymes that convert them, within seconds, into an irritating gas. The gas reaches your eyes, and your tear glands kick in to flush it away. A well-chilled onion or a very sharp knife limits the damage.
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.