Fluids are cool; they ooze and swoosh. Whatever container you put a fluid in, that fluid will take the same shape. Milk poured into a pitcher forms to the shape of the pitcher. If you pour it into a glass, it takes the shape of the glass. You just can’t do that with a boulder. But with the right container, you could pour liquid rock from below the Earth’s crust. Fluids still act like fluids, even halfway to the center of the Earth.