A zoetrope is a drum with a ring of pictures around the inside and a slit at the top. Spin it, look through the slit, and the pictures do not flicker — they run. The eye cannot resolve a change faster than about fifty times a second, so if the drum passes the slit often enough, the separate pictures are stitched into motion.
Drag the drum to spin it by hand. Peek removes the slit so you can see all the pictures at once — which is the whole trick, and the whole secret.
The zoetrope is attributed to William George Horner, a Bristol engraver, who published it in 1834 as a "daedaleum" — a mechanical toy that showed the illusion of motion. Three years later the Parisian optician SimonStampfer patented a version with a slitted cardboard drum, and it was a triumph at the Paris industrial exhibition of 1839.
The later phenakistiscope, spinning disc with painted figures seen through a hole, is the same idea flattened. And in 1889 Étienne-Jules Marey and Georges Eastman were both independently making the leap from the spinning drum to the strip of film.