An extensive study was conducted of students' explanations written in
response to 'what if...?' questions in elementary mechanics. The study
showed that the structure of students' explanations yields roughly th
e same ranking of students as do problem-solving tests, but in additio
n provides a wealth of insights into (1) context dependence and catego
rization in students' use of concepts, (2) the effect of misconception
s on context dependence, and (3) the types of explanations that studen
ts tend to produce. A follow-up study, in which students were presente
d with pairs of pre-written explanations to 'what if...?' questions an
d asked to indicate a preference, showed that students do not necessar
ily prefer the types of explanations they write, and have greater diff
iculty assessing the correctness of explanations that are counter to p
reference type. Evidence is presented that, for many students, the lin
ks between physics thinking and real-world thinking are all too tenuou
s.