hen astronauts on the International Space Station need to go number two, they direct their poo through a narrow hole into a carefully sealed toilet. Eventually, their waste bursts into flames when jettisoned into Earth’s atmosphere. The fate of the feces of 20 mice tagging along on the ISS this year won’t be quite as flashy, but it’s just as dramatic. The rodents, who shot into space on June 29, made a voyage to the station to provide scientists data on the effects of microgravity on their bodies and internal rhythms—part of which will be captured in their poop. Sound familiar? It should. In 2015, NASA did the same thing, but with people. The Rodent Research-7 study is a sibling of the Twins Study, during which astronaut Scott Kelly spent a year on the station while his brother, Mark Kelly, acted as a control back on Earth. Scientists have spent years poring over the data generated by the experiment—among them, the researchers who designed the mouse mission.