The rabbit-duck is a type of illusion called an ambiguous figure, and while we can’t know the ancient carver’s intent, this prehistoric knickknack is one too. According to Kyle Mathewson, a cognitive neuroscientist at the University of Alberta, the flip-flopping happens because the mammoth and bison share the same basic outlines. Your brain’s visual cortex makes sense of this low-level information, and then hands it off to a set of high-level processing neurons, which parses features like fur or tusks. This way, two different results stem from the same set of lines. Mammoth, bison; tomato, tomahto.