Series 4000 mechanoids were modular units, able to remove defective or malfunctioning parts and replace them with spares supplied by Divadroid International. Kryten had three spare heads which he kept in a storage locker aboard Red Dwarf. At one point, Kryten's Spare Heads had held a vote and decided that he was the big-eared, ugly one. (RD: Stoke Me A Clipper)