You are standing by the duck pond with two pails. One holds 7 gallons and the other holds 3 gallons. Neither one of them has gallon markings on the side. How can you get exactly 5 gallons of water in the big pail?
Fill 3; dump it into 7.
Fill 3; dump it into 7, which now has six gallons in it.
Fill 3; dump one gallon into 7 (you can do this, since it makes 7 full).
The 3 now has two gallons left in it.
Empty 7 completely; pour the two gallons from 3 into 7.
Fill 3; pour it into 7.
The 7 now has five gallons in it.