Swinging/Twirling Blind String

Earlier tonight our son was fixing the girls' beds and he seen the string from the blinds swinging and twirling. he came down and told me and then he said he was going to go back up and try to debunk it. He went up and did what he did before, thinking maybe it was wind from sheets and blankets being fixed. He could not get it to do it again after many attempts and he came back down and told me and that it sorta freaked him out a little.