Der Test (1964)

Two boys try out a home-made flying machine in their children's room at night. For another test, a rat that the boys keep as a pet is thrown out of the window with a small rocket. After this has also worked, the first boy stands on the windowsill ready to jump into the yard. At this moment, a man suddenly flies into the children's room from outside through the window, introducing himself as an inhabitant of Mars.

Unfortunately the movie Der Test is not yet available on HBO Max.