Conker
A Conker is the name used in Britain for the hard brown nuts of the Horse-Chestnut tree. Conkers are used in a game traditionally played by young children.
The Game
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 others 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 matchese 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. It is possible to harden them further by baking or soaking in vinegar overnight but this is widely viewed as cheating.
Fiction
The Conker as Hard as a Diamond by Chris Powling is a comical childrens book about a fantastical conker tournament and makes a moral point about not bragging about victories.
Controversy
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.