The rocket runs on extremely cold fuel. It's impossible to perfectly maintain that temperature in the tank, so the fuel is constantly expanding and venting out

The rocket runs on extremely cold fuel. It's impossible to perfectly maintain that temperature in the tank, so the fuel is constantly expanding and venting out of the tank so it doesn't explode. You can keep adding more chilled fuel to the tank though, so the goal is to top off the tank as close to launch as possible.