Before the invention of particle accelerators, the only source of particles moving close to the speed of light was cosmic ray showers. These are caused by high energy protons hitting the earth's upper atmosphere, producing a cascade of particles. Most of these deacay before reaching the surface of the earth, but muons, which are elementary particles rather like 'heavy electrons' (i.e. they have the same charge as and are more massive than electrons). However, they are unstable, and have a halflife of 1.5 microseconds, i.e. after 1.5 microseconds, half of the original population will have decayed.

Let's put the tortoise at the top of the mountain, the sparrow at the bottom, and give them a muon detector each. The mountain is 2000 m high, and the sparrow and tortoise agree that the muons take 6.7 microseconds to get from the top to the bottom. The tortoise measures muons arriving at a rate of 563 per hour.