Solution
Three Switches, One Bulb
Show the problem again
Three switches downstairs control one incandescent bulb upstairs; all switches are initially off, and you cannot see the bulb from the switches. You may manipulate the switches however you like. What is the minimum number of trips upstairs needed to determine with certainty which switch controls the bulb?
Worked solution
The answer is 1 trip. Turn switch 1 on for several minutes, then turn it off and turn switch 2 on, and go up: a lit bulb means switch 2, a dark-but-warm bulb means switch 1, and a dark, cold bulb means switch 3. The trick is realizing the bulb carries information in two channels (light and heat), so a single observation distinguishes three states.
Source: Classic switches-and-bulb deduction puzzle in wide circulation; no traceable original source. Statement written for AxiomIQ.