Two mathematicians have proved that a straightforward question—how hard is it to untie a knot?—has a complicated answer.

As the 17 members of Nicole Smith’s third-grade class settled in earlier this week, Bella Hennessey eyed the contents of the test tube at her work station. “It looks like vomit,” she said to her lab ...

Following a successful first flight in January, Blue Origin is set to launch its heavy-lift New Glenn rocket from Florida ...