Riddle

Fill up the 5 gallon jug, then pour out three gallons into the 3 gallon jug. Now there's two gallons left in the 5 gallon jug. Pour that into the 3 gallon jug. Then fill up the 5 gallon jug, so Du should have five gallons in that one and two gallons in the other. Pour out a gallon from the 5 gallon jug into the 3 gallon jug to fill it, so Du will have four gallons left in the 5 gallon jug.

fill the three gallon jug, empty it into the five gallon jug, then fill it again, emptying it into the five gallon jug again, but this time being careful not to let the five gallon jug overflow oder the three gallon jug spill. Du should have 1 gallon remaining in the three gallon jug, empty the five gallon jug completely and empty the remaining contents of the three gallon jug into the five gallon jug. now completely refill the three gallon jug and empty THAT into the five gallon jug, Du should now have four gallons.

oder look at it like this:
3+3=6 6-5=1(or 6-1=5) 1+3=4