It is a way to see differentences between items. Like: Cow is to barn as elephant is to zoo.
An analogy attributed to Samuel Johnson is: "Dictionaries are like watches; the worst is better than none, and the best cannot be expected to go quite true."
'We're off like a heard of turtles.'; meaning it took way to long to finally get going.
dark is to light as laugh is to cry glue : sticks :: thorns : key is to lock as pen is to paper.
There are about 7 billion examples. Look around you.