There are lots of little endings in this book of nine stories, but let's deal with the last one: Calvin and Byerley discuss how the Machines have started directing human progress. Byerley seems upset by this, but we think Calvin makes a pretty good case—after all, all of us humans are subject to forces that are beyond our control. Sure, we might increase our control over certain parts of our lives (yay penicillin and other antibiotics); but if you've ever had a cold ruin a day, you know that there are some things that are beyond our control even today.
But even so, this ending opens up a lot of questions. We mean, the whole book has stories about how we don't really need to worry about robots rebelling and killing us all and taking over. And then the last story ends with something that could be described as a robot take-over. It's the kind of ending that makes us ask questions—which is our favorite type of ending.
And after that, the narrator tells us that Calvin retires and dies, which is like a big hint to us that even though Susan acts pretty cold and robotic, she is only human after all.