Physicist Richard Feynman wondered what would happen if an S-shaped lawn sprinkler, which rotates as water squirts out, were placed underwater and had its flow direction reversed, so that it sucked water in. Which direction would it rotate? Experiments have given conflicting answers, but now researchers have provided what appears to be a definitive resolution. When sucking water in, the sprinkler reverses its rotational direction, and the motion is unsteady and much slower. The explanation involves the details of fluid flow in the sprinkler geometry.