In Death Valley, there is a place called Racetrack Playa where stones seem to move by themselves. Each stone moves a little bit every year, leaving a ________ on the ground. Scientists have been studying these "sailing stones" since the 1940s. They thought that wind, water, or ice might make the stones move.

In 2011, a scientist named Ralph Lorenz suggested that ice might lift the stones off the ground so the wind could move them. Recently, Lorenz and other ________ used cameras and GPS to watch the stones move. They discovered that the stones move when certain weather ________ happen.

First, it has to rain. Then, it has to freeze at night so that ice forms. The next day, the ice needs to break into large ________ and float on water. The wind can then push the ice into the stones, moving them across the ground.

Jim Norris, one of the researchers, said, "It's possible that tourists have actually seen this happening without ________ it." It is very ________ for all these conditions to happen at the same time. But when they do, the ice can move many stones at once. On December 20, 2013, more than 60 rocks moved on the same day.