A Conker is the name used in Britain and the United States for the hard brown nuts of the Horse-Chestnut tree. Conkers are used in a game traditionally played by young children.
A game of "conkers" is a game for two players. Playing conkers is a simple affair. Players will select a conker and make a hole through the centre of it. A piece of string is then threaded through the hole and tied off so that the conker freely hangs from the bottom. The opponents then face each other and take turns hitting each other's conker with their own whilst the string is held in one hand.
The winner is the first player to smash the other conker. Once a conker has successfully smashed another, it becomes a "one-er". With each successive victory, it adds a further number (e.g. a conker victorious in 6 matches would be a "six-er").
The trick to winning is to pick a large, hard conker. Children will gather many conkers in the autumn allowing them to select the best one.
Although many schoolboys through the ages have attempted methods of artificially hardening their conkers, this is widely viewed (especially in tournaments) as cheating. Popular methods have included varnishing the conker using clear nail polish, baking or microwaving the conker to remove moisture and soaking the conker in vinegar or household cleaner to harden it.
Using an old, unused conker from a previous year also gives the player an advantage. Once again, this is frowned upon although it is not explicitly cheating.
The Conker as Hard as a Diamond by Chris Powling is a comical children's book about a fantastical conker tournament and makes a moral point about not bragging about victories.
Although a game that has been played in schoolyards for hundreds of years some schools have considered banning the game due to fear of injury. This is a recent phenomena no doubt inspired by the new Health and Safety culture.