In orbit there is no G-Force (this is called free fall or "zero gravity"). To help prepare astronauts experience zero gravity, NASA trainers put the astronauts in water. Because water makes one float, this is a little like experiencing no gravity. However, in water they can push against the water and move around. In zero gravity, there is nothing to push against, so they just float in the air. Another way of training is going in a plane and making the plane fall to earth very quickly. This lets people experience zero gravity for a very short time. This training can make people quite sick at first.