Water

Answer: You fill up the 5 gallon jug and pour it into the 3 gallon jug. Then dump out the 3 gallon jug and that leaves you with 2 gallons in the 5 gallon jug. Pour the 2 gallons from the 5 gallon jug into the 3 gallon jug. Then fill up the 5 gallon jug and pour whatever you can into the 3 gallon jug, which is 1 gallon. That leaves you with exactly 4 gallons in the 5 gallon jug. MATH: 5-3=2. Put the 2 in the 3 jug. 5-1=4.