This article presents results of the first two years of the introduction of a critical thinking (CT) component to standard freshman astronomy lab exercises for nonmajors. The component consists of a series of probing questions folded into the exercises, plus a formal grading rubric. The grading rubric is adapted from the generalized Washington State University "Guide to Rating Critical Thinking" developed by the Washington State University Critical Thinking Project. The questions asked of the students are very specific applications of the more general "Guide". The motivations, implementation, and results of the use of this CT component are presented, as well as some ideas for further improvement. (Contains 1 table.)