This document discusses the idiom "Don't look a gift horse in the mouth" and how reading requires thinking. It explains that the idiom means it is foolish to inspect the mouth of a horse that was given to you for free. Looking in a horse's mouth allows one to determine its age and value, but inspecting a gift implies you are assessing the worth of something received freely. The document encourages readers to use vocabulary knowledge, prior knowledge, and knowledge of figurative language to understand idioms and think while reading, rather than just decoding words.