NASA astronaut Scott Kelly has returned to Earth after living for nearly a year aboard the International Space Station (ISS). When Scott Kelly arrived in Houston on Thursday morning, he was about two inches taller than when he left for the ISS a year before, according to NASA representatives.

What caused this growth? Well, it is very common among astronauts who spend a significant amount of time in space to gain a couple inches. It’s a weird but temporary side effect of spaceflight. Here’s how NASA explains it:

It is all because of the microgravity environment that allows their vertebrae to expand. Spending time in a zero-G environment, like that of the ISS causes the spine to stretch. On Earth, gravity keeps the vertebrae in place by constantly pushing them together. But without gravity, the vertebrae will naturally expand slightly, causing a person to become taller.

Imagine that the vertebrae in your back forms a giant spring. Pushing down on the spring keeps it coiled tightly. When the force is released, the spring stretches out. In the same way, the spine elongates by up to 3% while humans travel in space. There is less gravity pushing down on the vertebrae, so they can stretch out — up to 7.6 centimeters (3 inches).

However, Scott Kelly should return to his regular height after spending some time on the ground.

Scott Kelly has spent more time in space than any other American astronaut. One of the main goals of his groundbreaking mission is to study how well humans can endure — mind, body and spirit, on a long-duration spaceflight. One unique advantage he provides to NASA’s doctors is his identical twin brother, Mark Kelly, a former NASA astronaut who spent last year with his feet planted on terra firma.

Scott Kelly’s height change was especially striking when he stood to his identical brother, Mark Kelly. Each brother has undergone regular testing throughout the year-long experiment, and tests will continue as Scott Kelly readjusts to life on ground. Comparing the twins will help researchers spot any genetic changes that might have occurred in Scott in space.