First fill the 5-gallon jug, then pour the 5-gallon jug into 3-gallon one until completely filled. 2 gallons are left in the 5-gallon jug. Empty the 3-gallon jug and pour the 2-gallons of water into it. Now refill the empty 5-gallon jug completely and then pour that into the 3-gallon until full. What remains in the 5-gallon jug should be 4 gallons.
Joke:
How did the trout become a symbol for Christianity?
Answer:
Easy. By dropping “trou”