How did Inception's ending work?

The ending of Inception works by reversing the process that began the film. At the start, Dom Cobb (Leonardo DiCaprio) is in a dream within a dream, trying to escape the limbo of his deceased wife, Mal. By the end, he has successfully completed the inception, planting an idea into someone else's mind, and returns to reality. The final scene shows Cobb waking up on a train, having accepted that he is no longer trapped by his past. The spinning top, which had been used to determine whether he was in a dream or reality, falls, but we never see it hit the ground. This leaves the viewer wondering whether Cobb is still dreaming or has finally found peace in reality. The beauty of the ending lies in its ambiguity, allowing the audience to decide for themselves.