You have a 5 gallon jug and a 3 gallon jug.
You have no other measuring devices.
Measure out exactly 4 gallons of water (Assume you have a river handy, or some other limitless water supply).
First, you fill the 3 gallon jug to the brim. Then you pour that into the 5 gallon jug. Then, you fill the 3 gallon jug again to the brim. You then fill the 5 gallon jug to the brim with the 3 gallon jug, leaving you with 1 gallon in the 3 gallon jug. Then, completely dump out the 5 gallon jug, and pour the remaining gallon in the 3 gallon jug into the 5 gallon jug. Finally, fill-up the 3 gallon jug to the brim again and pour it into the 5 gallon jug.
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