The purpose of the study is to investigate how children and preservice teachers would make a progress in solving the open-problems related to area. In knowledge-based information age, information inquiry, information construction, and problem solving are required. At the level of elementary school mathematics, area is mainly focused on the shape of polygon such as square, rectangle. However, the shape which we need to figure out at some point would not be always polygon-shape. With this perspective, many open-problems are introduced to children as well as preservice teacher. Then their responses are analyzed in terms of their logical thinking and their understanding of area. In order to make students improve their critical thinking and problem solving ability in real situation, the use of open problems could be one of the valuable methods to apply.