On the morning of Aug. 6, a law professor at the University of Michigan wrote a tweet about a family that had been separated by immigration authorities after crossing into the United States.

Then the professor, Beth Wilensky, went for a run. Then she ate some lunch.

“I came back and checked my Twitter feed and said, ‘Oh my goodness,’” Ms. Wilensky recalled. Her post was blowing up, on its way to getting tens of thousands of retweets.

The tweet said: “My husband travels a lot. Downside: he’s gone a lot. Upside: frequent flier miles. We just used some to fly a 3-yr-old and his dad, who had been separated at the border, from Michigan (where the son had been taken) to their extended family. DM me if you have miles to donate.”

A “zero tolerance” immigration policy from the Trump administration led to the separation of thousands of children from their parents or guardians this year. In June, President Trump signed an order meant to stop the separations. But hundreds of children remain in United States custody without their parents, according to the Homeland Security Department.