Knowledge-in-action is a teacher's form of inherently knowing one's actions in the context of daily teaching activities. This article focuses on one physics teacher in a study and describes and analyzes his knowledge-in-action. Observations and interviews were used as data. Among the findings are the teacher's practice can be delineated into two modes, a classroom mode and a laboratory mode. Students in the classroom mode are expected to mentally reason as they are guided by the teacher. In the laboratory mode, the teacher did not hand out detailed instructions for performing labs as he expects students to develop procedures for themselves. The teacher treats Newton's laws of motion as unquestionable givens. (PR)