Freehand sketching is a fundamental skill in mechanical engineering and many other engineering disciplines. It not only serves as a communication tool among engineers, but plays a critical role in engineering design and problem solving. However, as computer-aided drafting has replaced traditional drawing classes nowadays, the training of students' freehand sketching skills has been almost completely eliminated in modern engineering curricula. This paper describes the attributes of freehand sketching and its roles in several essential aspects of engineering; in particular, in its roles in problem solving, of which current literature has ignored. Representative examples are provided to show students' freehand sketching skills in problem solving in a foundational undergraduate mechanical engineering course. Pedagogical suggestions are made on how to teach freehand sketching to engineering students.