Not everything in a text is clearly stated. When reading, we have to make links what we read and what we already know about the subject.
Read this brief text and try to answer this question:
Can metal detectors be used to trace water pipes under a street?
Metal detectors can find buried metal. They were developed for military purposes to locate hidden explosives. They operate on the principle of electromagnetic induction.
To answer the question, you have to link knowledge from the text and knowledge of your own.
From the text we know:
Metal detectors can find buried metal.
From your own knowledge you know:
Most water pipes are metal. They are buried under streets.
The more knowledge we have about our subject and about the world in general, the easier it is to learn new things when we read.