In grammar an apposition means a sentence or other grammatical construction composed by two elements that are located side by side and separated by another element that serves to make a difference between the two elements that are side by side, this third element is called appositive. As a consequence appositives are usually in the middle of the sentence or in the middle of the two elements that are in apposition and separated by commas or other symbols. In this case, <em>a scientist and critic</em> is the appositive as it is between the two elements in apposition <em>When Sandy met John </em>and <em>She fell in love with him</em>, establishes a difference or transition between the two elements and is separated by hyphens (-).
