This study aims to determine the cognitive and metacognitive strategies used by pre-service mathematics teachers for activating mental processes in a semi-structured problem-posing task. A holistic multiple-case design was used to in this study. For the case study, five voluntary pre-service mathematics teachers participated in this study. This task involves problem-posing in the context of science appropriate for different mathematical expressions. A think-aloud protocol, a semi-structured interview, observation and the pieces of papers for each question were used in this study. Open coding was performed using the continuous comparative analysis technique. The main results are that (a) they used various cognitive and metacognitive strategies to activate mental processes in problem-posing, (b) these strategies differed both in diversity and the usage of frequency of them and some strategies are either domain-specific or general-specific and (c) the use of metacognitive strategies is more common than cognitive strategies.