They had been working on the same problem for three hours, passing the laptop back and forth, one of them typing while the other talked through the logic. They had stopped noticing the handoff. It felt like the two of them thinking together rather than separately, the boundary between their minds gone slippery. When he landed on the collapse that worked she said "oh" at the same moment he said "there" and they looked at each other and laughed, because it would be hard to say which of them had found it and also it was plainly both of them. Neither was willing to take credit or give it up.