Imagine that it is possible to learn effort estimation concepts and its application in an attractive manner, where complex and technical knowledge are presented through a playful experience. The serious game Back to Penelope face this challenge, in which the student is trying to save the life of a young girl astronaut from a desolated world. To do this, the player must correctly estimate the development effort for the vital systems of the spaceship of the astronaut. The game is based on the COSMIC measuring method for estimation. This paper presents the results obtained from a systematic review performed to identify the main features of the existent serious games related to estimation. Moreover, this paper presents the design of the game developed, which has elements to improve the motivation of the students that use Back to Penelope. Finally, we present the promising results obtained from the validation of the game when teaching software engineering courses.