Osmium is the densest known element: about 22.6 grams per cubic centimetre, twice as dense as lead. A block the size of a milk carton would weigh as much as a six-year-old child. Its neighbour iridium comes an extremely close second. Very hard, osmium was once used to tip fountain-pen nibs.
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.