Ok, here's an easy one that most people probably know...but im too lazy to come up with anything else
You have a 5 gallon jug and a 3 gallon jug...fill one of the jugs with exactly 4 gallons of water. You can only use the two jugs you have (3 & 5 gallon)
-Darth Simon
p.s. bonus points to whoever can tell me what movie this riddle is used in