The waving flag

In the still photos the flag is not hanging limp; it is spread out with waves in it. In the film it swings back and forth as the astronauts plant it. On Earth those are the signs of wind, and a sealed studio with air-conditioning would produce exactly that. [1]

NASA knew a flag would hang limp in a vacuum, so the design added a horizontal telescoping rod along the top. On Apollo 11 the rod did not fully extend, which left the nylon bunched in ripples. The crews later liked the look and left it that way. [1][2]

The flag moves only when the astronauts are touching or twisting the pole. Without air to damp it, the swing lasts a little longer than on Earth, then stops. Watch the full sequence, not a cropped clip, and you see the motion begin and end with the astronauts’ hands. [3]

In 2008 the MythBusters team took a replica flag into a NASA vacuum chamber at Marshall Space Flight Center and twisted it. It moved the same way. [4] Even Soviet cosmonaut Alexei Leonov explained the effect this way in 2009. [5]