The Next-Generation Science Standards (NGSS) call for a different approach to learning science. They promote three-dimensional (3D) learning that blends disciplinary core ideas, crosscutting concepts and scientific practices. In this study, we examined explanations constructed by secondary science teacher candidates (TCs) as a scientific practice outlined in the NGSS necessary for supporting students' learning of science in this 3D way. We examined TCs' ability to give explanations that include explicit statements of underlying reasons for natural phenomena, as opposed to simply describing patterns or laws. In their methods courses, TCs were taught to organize explanations into a what/how/why framework, where what refers to what happens in specific cases (data or observations); how refers to how things usually happen and is equivalent to patterns or laws; and why refers to causal explanations or models. We examined TCs' ability to do this spontaneously and in a resource-rich environment as a first step in gauging their preparedness for NGSS-aligned teaching. We found that (1) the ability of TCs to articulate complete and accurate causal scientific explanations for phenomena exists along a continuum; (2) TCs in our sample whose explanations fell on the upper end of this continuum were more likely to provide complete and accurate explanations even in the absence of support from explicit standards; and (3) teacher candidate's ability to construct complete and accurate explanations did not correlate with cross-course performance or academic major. The implications of these findings for the preparation of teachers for NGSS-based science instruction are discussed.