In the 17th century, Antonie van Leeuwenhoek used one of the earliest microscopes to see how sperm swim. He described the sperm as a “living animalcule” with a “tail, which, when swimming, lashes with a snakelike movement, like eels in water”. Strikingly, this perception of how sperm moves has not changed since. Indeed, anyone today with a modern microscope would make the same observation: sperm swim forward by wiggling their tail symmetrically side-to-side. Our new research using 3D microscopy shows that we have all been victims of a sperm deception, an illusion. Only now we can see that for 350 years we have been wrong about how sperm actually swims.