This study focuses on adults’ geometric knowledge specifically related to triangles. Two questionnaires were filled out by 148 adults. The first questionnaire presented two examples and three nonexamples of triangles. Adults were asked to identify each figure as a triangle or non-triangle and explain their reasoning. The second questionnaire contained the same figures and questions, along with the responses of a fictitious five-year-old boy. Adults were requested to evaluate the boy’s identifications and reasoning. Except for one non-triangle, nearly all adults correctly identified the figures and offered geometrically sufficient explanations. Adults tended to accept the young boy’s reasoning even though it was not based on critical attributes, claiming that for young children, such reasoning is acceptable.