You have 2 pails. One holds 4 gallons and the other holds 7 gallons. How can you go to the lake and bring back exactly 5 gallons of water?

fill up the 4 and dump it in the 7. then fill up the 4 again and dump it in the 7. you will have 1 gallon left in the 4. then dump out the 7 and dump the 1 from the 4 in the 7. fill up the 4. 4 + 1 = 5!