  The traditional approach to studying student understanding presents a
question and uses the student answers to make inferences about their knowledge.
However, this method does not capture the range of possible alternative ideas
available to students. We use a new approach, asking students to generate a
plausible explanation for every choice of a multiple-choice question, to
capture a range of explanations that students can generate in answering physics
questions. Asking 16 students to provide explanations in this way revealed
alternative possibilities for student thinking that would not have been
captured if they only provided one solution. The findings show two ways these
alternatives can be productive for learning physics: (i) even students who
ultimately chose the wrong answer could often generate the correct explanation
and (ii) many incorrect explanations contained elements of correct physical
reasoning. We discuss the instructional implications of this multiple-choice
questioning approach and of student alternative ideas.
