Fans of Die Hard With A Vengeance will remember this one: To disable a bomb, Detective John McClane must measure out exactly 4 gallons of water, and place the resulting weight on a scale. His tools are yours: a 3-gallon and a 5-gallon jug—and a single fountain. McClane did it in less than 5 minutes, but we won't time you. This go-round, lives probably aren't on the line.
A) Fill the 5-gallon jug. Use it to fill the 3-gallon jug, leaving 2 gallons in the 5-gallon jug. Pour in those 2 gallons. Fill the now empty 5-gallon jug and pour 1 gallon into the 3-gallon jug. There are 4 gallons left in the 5-gallon jug.
B) Fill the 3-gallon jug and pour it into the 5-gallon jug. Fill the 3-gallon jug again and use it to fill the 5-gallon jug. You have 1 gallon. Empty the 5-gallon jug and pour the 3-gallon jug's remaining gallon in. Refill the 3-gallon jug and pour it into the 5-gallon jug.
This article was originally published in the Summer 2019 Make It Last issue of Popular Science.