Did Earth get its water from the late-stage accretion of comets? Or did it get it during the accretion process? If it was accreted, could Earth have kept its water after a giant Moon-forming impact? Using high-precision oxygen isotope measurements, a team led by Richard Greenwood from the Open University showed that lunar rocks and terrestrial basalts are statistically distinct. The results indicate that between 70 and 95% of Earth’s water was accreted before the giant, Moon-forming impact.