Gravity rules the planets, yet its constant, written G, is surprisingly hard to pin down. Between two lab objects, the pull is so weak that the slightest disturbance skews the result. G is known only to a few tens of parts per million, while other fundamental constants are known a thousand times better, or exactly. Since Cavendish's experiment in 1798, measurements still struggle to agree.
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.