This study employed a pretest-posttest control group experimental design to investigate the impact of self-explanation prompts on content learning in the game-based context. Additionally, it explored the effectiveness of different delivery timings for the prompts--either during or after gameplay--in enhancing students' cognitive engagement in self-explanation and content learning. The physics educational game "Save Kitten" was developed using Articulate Storyline 360®, incorporating in-game textual self-explanation prompts that asked students to explain their game solutions based on targeted physics knowledge. Students voluntarily responded to these prompts by drawing force diagrams on a handout. The participants consisted of 92 10th-grade students from a large public high school in southern China, who were randomly assigned to one of three conditions: "control" (playing the game without prompts), "during" (playing the game with prompts delivered between game levels), and "after" (playing the game with prompts delivered after completing gameplay). Quantitative data were collected through a demographic questionnaire, pretest and posttest assessments of physics knowledge, and a handout for written self-explanation. Descriptive analysis, correlation analysis, multiple regression, Chi-squared test, independent Mann-Whitney U test, paired-sample t-test, one-way ANOVA, and one-way ANCOVA were performed using SPSS version 25.0 to analyze the quantitative data. Qualitative data collected through in-field observations and informal student interviews were thematically analyzed manually. The quantitative findings revealed that students who played the game with self-explanation prompts, either during or after gameplay, achieved significantly higher scores in the posttest compared to those without prompts after controlling for pretest scores. Moreover, students demonstrated better learning outcomes when more engaged in self-explanation. There was no significant difference in self-explanation engagement between students who received the prompts at different timings. However, during-gameplay prompts for self-explanation proved more effective than prompts delivered after gameplay in enhancing students' content learning through gameplay. Qualitative findings indicated that students displayed a strong level of involvement and interest in gameplay, compared to physics learning, and a collective focus on game achievement. Some students who were not prompted demonstrated spontaneous self-explanation, while others who received prompts did not engage in self-explanation as anticipated. Recognizing the instrumental value of self-explanation could encourage students to engage with the prompts, whether in written form or mentally. This study provides empirical evidence supporting prompting self-explanation when students play educational games and offers insights into effective strategies for designing and implementing the self-explanation prompts in game-based learning contexts. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]