A bartender has a three pint glass and a five pint glass. A customer walks in and orders four pints of beer. Without a measuring cup but with an unlimited supply of beer how does he get four pints in either glass?

These classic measurement problems challenge us to think creatively about how to use fixed capacities to achieve a desired amount. One effective way to measure four pints involves first filling the five-pint glass completely. By then pouring from the five-pint glass into the three-pint glass until the latter is full, exactly two pints are left behind in the five-pint container. Emptying the three-pint glass and transferring those two pints into it sets the stage. Finally, refilling the five-pint glass and topping off the three-pint glass (which already holds two pints) will remove precisely one pint from the larger glass, leaving exactly four pints remaining.

An equally valid alternative begins by filling the three-pint glass and transferring its contents to the five-pint glass. Repeating this process, the three-pint glass is filled again. When this second three-pint measure is used to fill the five-pint glass completely, one pint is left over in the smaller three-pint glass. This single pint is crucial; it is then poured into the now-empty five-pint glass. A final step of filling the three-pint glass once more and adding its contents to the five-pint glass, which already contains one pint, successfully yields the requested four pints.

These intriguing challenges are classic examples of "water pouring puzzles" or "jug problems," often found in recreational mathematics and logic tests. They require creative thinking about how to use the available containers to measure precise quantities, relying on the differences and sums of their capacities. Such puzzles have captivated thinkers for centuries, appearing in various forms from ancient riddles to modern brain teasers, demonstrating their enduring appeal as tests of logical deduction and problem-solving skills. They highlight how seemingly simple tools can be used in complex ways to achieve specific outcomes.