You have a 4 gallon and a 7 gallon jug of water. How can you measure exactly 6 gallons of water using these two jugs?
Fill up the 7 gallon jug completely. Now pour as much of the 7 gallons as you can into the 4 gallon jug. You should have 3 gallons in the 7 gallon jug. Dump out the 4 gallon jug, pour the 3 gallons from the 7 gallon jug into the 4 gallon container. Finally, fill up the 7 gallon jug completely and pour what you can, 1 gallon, into the 4 gallon jug until it is full. Taking 1 gallon from seven gallons leaves 6 gallons.
What are four things that have an "eye" but cannot see?