the Doctor nevertheless referred to them as "the finest soldiers in the galaxy". Although physically formidable, the Sontarans' weak spot is the "probic vent" at the back of their neck, through which they draw nutrition. It is also part of their cloning process. In Earthlike atmospheres, the Sontaran armour suit requires a change of gasses every 27 hours, also conducted through the probic vent. The vent also provides an incentive to continue moving forward in battle since retreat would expose this area to their enemies. They have been killed by targeting that location with a knife (The Invasion of Time), a screwdriver ("Shakedown: Return of the Sontarans"), and an arrow (The Time Warrior). Even something as simple as a squash ball aimed at that point ("The Sontaran Stratagem"), contact by the heel of a shoe ("The Last Sontaran"), or a blow from a wok ("War of the Sontarans") is capable of incapacitating them temporarily. They are also vulnerable to "coronic acid" (The Two Doctors). While the Sontarans wear protective helmets in battle, to fight without their helmets, or to be "open-skinned," is an honour for the Sontarans. In the episode "The Poison Sky", it is revealed that the Sontaran Empire have been at war with the Rutan Host for more than 50,000 years, and which, at a time around 2008, they are losing. The war is still raging at least 20,000 years later, in the serial The Sontaran Experiment. It was joked that the two species have been at war for so long, they had almost forgotten the reason why. ("The Two Doctors") Most of the Sontarans depicted in the television series have had short names, many beginning with an initial 'st' sound. Examples include Styre (The Sontaran Experiment), Stor (The Invasion of Time), Stike (The Two Doctors), Staal ("The Sontaran Stratagem" / "The Poison Sky"), Stark ("The Pandorica Opens"), Strax and Stenck ("The Vanquishers"); others are Skorr ("The Sontaran Stratagem" / "The Poison Sky"), Ritskaw and Kragar ("The Halloween Apocalypse"), Skaak ("War of the Sontarans"), Linx (The Time Warrior), Varl (The Two Doctors), Jask ("The End of Time") and Kaagh (The Sarah Jane Adventures)). Elements of the Sontaran military structure mentioned in the series include the "Sontaran G3 Military Assessment Survey" and the "Grand Strategic Council", the Ninth Sontaran Battle Group, the "Fifth Army Space Fleet of the Sontaran Army Space Corps", and the "Tenth Sontaran Battle Fleet". Military