A high-velocity cloud (HVC) is a fast-moving interstellar cloud, composed mostly of neutral hydrogen, whose velocity is too high to be explained by the ordinary rotation of the Milky Way Galaxy. Many HVCs are thought to be material falling into our galaxy from the outside; one sign of an extragalactic nature is their heavy element content, which appears to be poorer than the Milky Way norm. They may be material left over from the formation of the Galaxy or that of other galaxies in the Local Group, or they may consist of matter wrenched away from a satellite system. A spectacular example of tidally-displaced debris is the Magellanic Stream.