Perceptual psychologists have long been interested in limen – the threshold at which a stimulus becomes detectable. The following limen for the different senses, expressed in everyday terms rather than in terms of physical quantities, have a certain poetry to them. I got this information via email as a scan of an (unknown to me) textbook. I reproduce them here for your enjoyment:
Approximate absolute sensitivities, expressed in everyday terms:
Vision – A candle flame seen at 30 miles on a dark, clear night
Hearing – The tick of a watch under quiet conditions at 20 feet
Taste – One teaspoon of sugar in two gallons of water
Smell – One drop of perfume diffused into the entire volume of a three-room apartment
Touch – The wing of a bee falling on your cheek from a distance of one centimeter
Exact values vary between individuals and even from moment to moment with the same individual. Source: Galanter, E. (1962). Contemporary psychophysics. Holt, Rinehart, Winston.