Emphasis on interdisciplinary thinking has resulted in the development of new courses in natural sciences aimed at bridging disciplinary barriers. Initiatives like introductory physics for life science (IPLS) and Chemistry, Life, Universe and Everything (CLUE) curriculum are exemplars in this regard, at the undergraduate level. However, more work is required on assessing interdisciplinary thinking. Lack of substantial precedents and vagueness on what does it mean to be interdisciplinary make assessment development in this area a challenging task. We consider invoking ideas from different disciplines and integrating them as indicative of interdisciplinary thinking in students and adopt them as criteria for assessing interdisciplinary thinking. Our study operationalizes these criteria based on textbook analysis, the details of which will be discussed in this poster. The data of our study constitute student explanations of a set of carefully chosen everyday, interdisciplinary phenomena such as what happens to an egg white on boiling. The disciplinary roots of the ideas invoked by students are traced by mapping them to introductory level textbook presentations. Knowing the disciplinary roots of knowledge elements underlying an explanation provides a picture of the extent of integration of ideas from different disciplines achieved in the explanation.