Your brain merges two sounds arriving less than about a tenth of a second apart. In a tenth of a second, sound covers roughly 34 metres, so an obstacle must be at least roughly 17 metres away for the round trip to produce a distinct echo. Any closer, and the reflection blends into your voice as mere reverberation.
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.