# Droppings
#
# If you went outside and lay down on your back with your mouth open, how long
# would you have to wait until a bird pooped in it?
#
# https://what-if.xkcd.com/11/

@aliases(birds)
unit bird
unit mouth
unit poop

let earth_radius = 6371 km

let frequency = (300 billion birds / 4 pi earth_radius^2) × (1 poop / bird / hour) × (16 hours / day) × (1 mouth / poop) × (15 cm^2 / mouth)
let period = 1 / frequency

print("It happens once every ~{period -> year}s")