In this study, we probed high school students' understanding of Einstein's theory of gravity by implementing an approach which mainly consists of two steps: firstly, exposing students to TEs describing the Equivalence Principle; secondly, applying the analogy of parallel lines on a curved surface with the path of two falling balls in a real gravitational field to help students deduce the idea of gravity as the spacetime curvature. A total of 12 high school students voluntarily participated in this study where data regarding their understanding were measured by means of an identical pen-and-paper test and interviews. Even though none of the students could explain what the Einstein's version of gravity is in the pre-test, their responses in the post-test indicated that the approach we applied could help them understand the Einstein's theory of gravity. Not only could they recall what the gravity is, most of them managed to provide related analogy they have learnt to explain their thoughts. Apart from its easily comprehensible steps, the study suggested that the approach is worth adopting to teach Einstein's theory of gravity as it reflects the similar path ever taken by Einstein when starting to formulate his theory of gravity.