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).
(In reply to
Another Way by randy)
I guess that you could really measure out any whole number of gallons, really with a 5 and 3 gallon combo.