Why one more candle lights a dark room but vanishes in a bright one

Add one candle to a dark room and everything changes. Add one to a room with a hundred and nobody notices. Your senses don't measure differences in amounts; they measure differences in ratios.

That rule, Weber's law, was spotted in the 1830s and still holds for brightness, weight, sound and even how many things you see at a glance. Typical adults can tell 10 dots from 11 in a flash, but not 100 from 101. Test your own number sense below, then see the same rule steer what people will drive across town to save.