There are several ways of creating such a transition, and these ways, basically, define the three main types of morphing: 1. Direct morphing. This is the type of morphing in which the shape of an object is directly transformed into another shape, without any cheating or ambiguity. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.