An example of a system with discrete symmetry is given by the figure with the red graph: consider a particle moving on this graph, subject to gravity. A similar graph could be given by the function f ( x ) = ( x 2 − − a 2 ) 2 . This system is symmetric under reflection in the y-axis. There are three possible stationary states for the particle: the top of the hill at x = 0 , or the bottom, at x = ± ± a . When the particle is at the top, the configuration respects the reflection symmetry: the particle stays in the same place when reflected. However, the lowest energy configurations are those at x = ± ± a . When the particle is in either of these configurations, it is no longer fixed under reflection in the y-axis: reflection swaps the two vacuum states.