The separation of uranium requires the material in a gaseous form; uranium hexafluoride (UF) is used for uranium enrichment. Upon entering the centrifuge cylinder, the UF gas is rotated at a high speed. The rotation creates a strong centrifugal force that draws more of the heavier gas molecules (containing the U-238) toward the wall of the cylinder, while the lighter gas molecules (containing the U-235) tend to collect closer to the center. The stream that is slightly enriched in U-235 is withdrawn and fed into the next higher stage, while the slightly depleted stream is recycled back into the next lower stage.