Gravitational lensing occurs when light travels past a very massive object. According to Einstein's theory of general relativity, mass shapes spacetime and space is curved by massive objects. Light traveling past a massive object follows a "straight" path in the curved space, and is deflected as if it had passed through a lens. Strong gravitational lensing can cause stars to appear as rings as their light travels in a curved path past a massive object along the line of sight. We observe microlensing when an object such as a MACHO moves between the Earth and a star. The gravitational lens associated with the MACHO focuses the star' light, so we observe the star grow brighter then dimmer as the MACHO moves across our line of sight to the star.