When Joy and Sadness first met Bing Bong, Riley's childhood imaginary friend, he was upset over the fact that his rocket wagon was dumped into the bottom of the pit, and was deeply saddened by this.
When Joy, Sadness and Bing Bong managed to hitch a ride onto the Train of Thought, the destruction of Honesty Island destroyed the train track, and the train began to fall. Joy and Bing Bong fell into the Memory Dump, but Sadness managed to avoid falling in. With the two heroes trapped at the bottom of the pit, all hope seemed lost. It is at this moment that Joy realizes something she had never noticed before: all the memories that were created by Sadness not only made Riley cry, but it also signaled others that she needed help. Realizing that Sadness' role in Riley's life is truly important, Joy declares that they have to get back up there.
Luckily, Bing Bong finds his rocket wagon, and in order to activate it, the two have to sing a special song. Joy and Bing Bong begin to sing the song as loud as they can, but no matter how loud they sing, they come up short of the top of the cliff. With one more attempt, they fly upwards, and while they do so, Bing Bong intentionally jumps off to make the wagon lighter, and Joy makes it. Seeing what Bing Bong did, Joy looks into the dump, and Bing Bong asks Joy to "take Riley to the moon for him," and he fades away, forgotten.