We have developed and verified laboratory work as guided inquiry for upper secondary level students, focusing on conceptual understanding of the physical principle that forms the basis of temperature measurement, and on improvement of selected skills. Conceptual pre-test questions initiate the students' interest and help identify input misconceptions. Using the method of interactive lecture demonstration, the students are introduced to the measurement principles of the Galileo thermometer. The students are then set the problem of how to repair a broken thermometer when tap water is used instead of ethanol. Since the density of water is greater than that of ethanol, the buoys must be adjusted by the students to achieve correct temperature measurement. The next steps of the activity have a hands-on orientation. The students work in pairs, guided by worksheet instructions. At the end of the activity, they complete self-assessment rubrics focused on skill improvement and final conceptual understanding. The results of the conceptual pretest questions and of the self-assessment rubrics from 461 participants are analysed and recommendations are made for teachers.