In which sport do you use a shuttlecock?

Badminton, and it is the fastest racket sport in the world by a distance - smashes have been recorded above 400 kilometres an hour, faster than anything in tennis, golf or squash. The shuttlecock is also the reason the game cannot be played outdoors seriously: it is so light and so affected by air that a slight breeze ruins it.

Its behaviour is unlike any other projectile. The conical skirt of feathers creates enormous drag, so it decelerates violently after leaving the racket - it starts faster than a tennis ball and arrives slower - and it always turns to fly cork-first whatever way it is struck. That deceleration is why rallies work: a shot that leaves at 400 km/h can still be returned.

The feathers are traditionally sixteen from the left wing of a goose, all from the same side so they curve consistently and spin the shuttle in flight. That is a real supply chain - top-level play still uses feather shuttles, which are fragile enough that a single match can go through dozens - and synthetic replacements have taken decades to approach the flight characteristics, which is a surprisingly hard aerodynamics problem.