I have a five and a three gallon jar.

Assuming there us an infinite supply of water , how can i measure one gallon water ?

Easy Gallon Riddle

-> Fill up the 3 gallon jar first , Pour it into the five gallon jar.
-> Fill up the 3 gallon jar again, and fill up the rest of the 5 gallon jar,
=> and you should have 1 gallon left over in the 3 gallon jar.