Sometimes, when two atoms collide against each other they fuse and become a single atom with a higher mass. This process does not happen easily yet it is faster than a blink of an eye. It requires a lot of efforts to reach the proper conditions: temperature, pression, kinetic energy and steric requirement, all of this must convey simultanely into a specific state in order to make the fusion possible.

If we think about the human soul as one of the atoms, there is a small chance the fusion could happen; some call it "meeting a soulmate". Now what makes two souls merge and become one before anyone can tell how it happened? That's not an ordinary happening. It's a nuclear fusion.