Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: As the atoms approach one another, their electron clouds gradually begin to overlap. Now there are several interactions which begin to occur. One is that the single electrons that each hydrogen atom possesses begin to repel each other. This repulsive force would tend to make the potential energy of the system increase. However, the electron of each atom begins to be attracted to the nucleus of the other atom. This attractive force tends to make the potential energy of the system decrease. Answer: overlap
Output:
As the atoms approach one another, their electron clouds gradually begin to do what?