How can the exact same face look hungry, grieving or in love?

Because your brain fills it in. In the 1910s and 1920s, the Russian filmmaker Lev Kuleshov took a shot of the expressionless face of Ivan Mosjoukine, a famous star of Tsarist cinema, and alternated it with other shots: a bowl of soup, a girl in a coffin, a woman on a divan. Audiences were sure the actor's expression changed each time, showing hunger, then grief, then desire. It was the very same shot of his face every time.

This is the Kuleshov effect: two shots in a row carry more meaning than either shot alone. Viewers bring their own feelings to the sequence and then credit them to the actor, as if his blank face were doing the acting. According to his collaborator Vsevolod Pudovkin, the audience raved about the performance. For Kuleshov, the lesson was that cinema's power lies less in what each image shows than in how images are combined, an idea that became the basis of Soviet montage cinema.

Alfred Hitchcock loved demonstrating it. Show him squinting, then a woman with a baby, then his smile, and he is a kind old man. Swap the baby for a woman in a bikini, and the very same smile makes him, in his words, a dirty old man.

The original experiment is hazy: Kuleshov and Pudovkin told different versions, and no film of it is known to survive. But modern studies, including brain scans in 2006, confirm the core idea. The context around a face changes how we read it.