Cybermen are another enemy of the Doctor. They are one of their worst enemies. In the original series, they come from the planet Mondas. In the new series, some of them come from a parallel universe. Cybermen travel across the universe taking people and turning them into machines. They stomp around saying "DELETE!". They kill by electrocution. They are partially vulnerable to the metal gold, and can be killed by gold bullets. They change their look over time, but stay recognizable by "handlebars" on their heads, and tear drops in their eyes.