This week, Claire tells us about new evidence for the leading theory of how the Moon was formed, through a collision of the early Earth with the Mars-sized protoplanet Theia; and Catriona updates us on where water can be found in our Solar System, and where it may have come from. Qian Yuan et al. 2023. Moon-forming impactor as a source of Earth’s basal mantle anomalies. https://doi.org/10.1038/s41586-023-06589-1