Have you ever hung upside down for a little while? Remember how it felt to have all the blood rush to your head? When astronauts get to space, they experience a very similar sensation, which they nickname "puffy head bird legs."

Astronauts have coined the term "puffy head bird legs" to describe this sensation. The cause of this phenomena is the movement of fluid within our bodies. Due to the lack of gravity in space, the extra gallon and a half of liquid that we carry on Earth to counteract gravity moves from our legs upward, like when you flip upside down. This creates a puffier head and thinner legs.