This visual demonstration was used in Heider and Simmel's classic experiment. (Video from youtube by Michael Smith-Welch. Thanks.)
[Heider, F., & Simmel, M. (1944). An experimental study of apparent behaviour. American Journal of Psychology, 13.]
According to them, most of the observers attribute some sort of "human-like agency" to the geometrical figures and imagine a story about the circle and the little triangle "being in love". And the big and "bad" triangle tries to "steal away" the circle.
Why do we naturally attribute such an agency or a sort of intentionality to these figures? Because their movements are similar to those of humans? Or, because of the whole pattern of stories? We can project a certain story onto the relation between three geometrical figures, which is accordance with human social behaviors?
Neither would be true. (1) We naturally find the intentionality to the animals' movements that are not similar to humans. (2) We also naturally see the intentionality to the infants who still cannot behave in a social way.
Then, how do you explain?
I found that Shaun Gallagher tried to give an account to this experiment, from the "interactionist" point of view;
I perceive the movement as something with which I could interact to some end. One could easily picture a larger scale virtual reality where I, as a human subject, am in the scene with the geometrical figures, and where I could intervene, play the game in a meaningful way, so to speak, for example, to prevent one figure from 'chasing' another. This possibility for intervention on my part is what I see in their movement as meaningful, and what constitutes the basis for my attribution of intentionality.
[Gallagher, S. (2012). Phenomenology. Palgrave-macmillan, p. 79]