Engineering students use spatial thinking when examining diagrams or models to study structure design. It is expected that most engineering students have solidified spatial thinking skills during K-12 schooling. However, according to what we know about geometry learning and teaching, spatial thinking probably needs to be explicitly taught within the confines of engineering-specific contexts in college. The van Hiele theory of geometry learning explains geometry understanding as a series of more and more sophisticated ways to reason geometrically. The theory is known for its use in guiding K-12 geometry instruction. This paper describes the theory and explains how one engineering mechanics professor used it to re-conceptualize and restructure his approach to teaching an engineering mechanics class. In particular, we describe his use of the van Hiele theory to move students toward success with freebody-diagrams, diagrams requiring complex spatial thinking and often a "point of departure" for most undergraduate engineering students. [A bibliography and list of suggested readings are included.]