There is a need to integrate the segregated perspective underlying research on scientific conceptions. Insights from scientists can provide information about the essential components of ideal knowledge. The purpose of this study was to investigate how researchers and teachers deal with scientific explanation. Three research physicists and five secondary physics teachers were asked to explain the Newton's Cradle demonstration. Written answers and follow up interviews were analyzed. All the respondents viewed the events as a series of collisions and related the phenomenon to the concepts of energy and momentum; however the arguments proposed as explanations differed in depth and in complexity. Results suggest that the differences in performances were related to: (1) the perceived purpose of the explanation and its nature; (2) the number of paradigms invoked for possible ways to describe the events; (3) the specification of assumptions underlying facts or data statements; (4) the examination of assumptions made to determine initial conditions; (5) the choice of variables and unknowns; (6) the proper application of scientific principles; and (7) the assessment of the entire argument in view of the acceptability of the underlying model and assumptions. Contains 29 references. (Author/LZ)