This study aimed to present a project-based STEM activity titled "Let's Build a Mini Farm on Our Schoolyard." The activity was implemented in science and science applications courses during the 2018-2019 academic year with 28 seventh grade students attending a state school in Giresun/Turkey. At the beginning of the study, the students learned about the engineering design process and project-based learning. Then, they were challenged with building a mini chicken farm on their schoolyard, which could be a solution of a real-world problem. To complete the project, they carried out mini researches and mini designs within the unit "Reproduction, Growth, and Development of Plants and Animals." They used their knowledge and experience in STEM disciplines to design the farm. The students carried out the project activities, offered solutions to the real-world problem, selected one of these solutions, tested the solution, and solved the problem with a sustainable method.