A single match can light up a room the size of a football stadium. If a killer is lurking in your house, it's easy to find him. Just relax and run a bath - even if it's the middle of the afternoon. Medieval peasants had perfect teeth. Although in the 20th century it is possible to fire weapons at an object out of visual range, people in the 23rd century will have lost this technology.
All single women have a cat.
Any person waking from a nightmare will sit bolt upright and pant. Even when driving down a perfectly straight road, it is necessary to turn the steering wheel vigorously from left to right every few moments. One man shooting at 20 men has a better chance of killing them all than 20 men firing at one. Creepy music coming from a graveyard should always be closely investigated. Most people keep a scrapbook of newspaper cuttings - especially if any of their friends or family have died in a strange boating accident.