Logic Puzzles

Step 1: Fill the 3-gallon jug and pour it into the 5-gallon jug.
Step 2: Fill the 3-gallon jug again. Pour from it into the 5-gallon jug until the 5-gallon jug is full. Since it already had 3 gallons, you pour in 2 more gallons, leaving 1 gallon in the 3-gallon jug.
Step 3: Empty the 5-gallon jug and pour the remaining 1 gallon from the 3-gallon jug into the 5-gallon jug.
Step 4: Fill the 3-gallon jug once more and pour all 3 gallons into the 5-gallon jug, which already contains 1 gallon. You now have exactly 4 gallons in the 5-gallon jug.