Output All nine dots crossed without lifting the pen Input Nine dots in a three-by-three grid to be crossed with four connected straight lines Four lines solve the nine-dot puzzle by extending beyond the grid SolvesGeneral
Output Choose the opposite door to find the safe route Input Two guarded doors, one safe, with one guard always truthful and one always lying One question defeats a liar and a truth-teller SolvesGeneral
Output Doors 1, 4, 9 through 100 remain open Input One hundred closed doors toggled on passes numbered 1 through 100 The hundred doors puzzle leaves perfect squares open RevealsGeneral
Output All disks transferred legally in the minimum seven moves Input Three different-sized disks stacked largest to smallest and two empty pegs Three Tower of Hanoi disks require seven moves SolvesGeneral
Output Switching wins with probability two thirds Input Three doors, one prize, an initial choice and a host who always reveals a goat Switching doors wins two thirds of Monty Hall games SolvesGeneral
Output The disturbance reaches a given underwater distance sooner Input The same sound-frequency disturbance moving through air and liquid water Why does sound travel faster in water than in air? AnswersGeneral
Output An earthy scent known as petrichor Input Rain beginning after a dry period on soil or rock Why can rain produce an earthy smell? AnswersGeneral
Output The warmer parcel moves upward until forces and temperature differences change Input A parcel of air warmed relative to surrounding air Why does warmer air tend to rise? AnswersGeneral
Output An approximately spherical bubble Input A flexible soap film enclosing air away from nearby surfaces Why are free soap bubbles round? AnswersGeneral
Output Eyes produce tears that dilute and wash away the irritant Input An onion cut open near unprotected eyes Why do cut onions make eyes water? AnswersGeneral
Output Exactly 4 litres remain in the 5-litre jug Input An unmarked 3-litre jug, an unmarked 5-litre jug and unlimited water Measure exactly four litres with two jugs SolvesGeneral
Output The heavier ball identified within two weighings Input Eight identical-looking balls with exactly one heavier ball Find one heavier ball in two weighings SolvesGeneral
Output Everyone crosses in 17 minutes Input Four people taking 1, 2, 5 and 10 minutes, one torch and a two-person bridge Cross the bridge in 17 minutes SolvesGeneral
Output All three boxes correctly labelled Input Boxes labelled Apples, Oranges and Mixed, with every label wrong Correct three mislabeled fruit boxes with one sample SolvesGeneral
Output Exactly 45 minutes measured Input Two ropes that each take 60 minutes to burn but burn unevenly Measure 45 minutes with two uneven-burning ropes SolvesGeneral
Output A star that appears to shimmer Input Light from a distant star passing through Earths atmosphere Why do stars twinkle? AnswersGeneral
Output A delayed repetition of the original sound Input A short sound made near a large hard surface Why does an echo return? AnswersGeneral