The answer, in the case of teenagers at the London Oratory, the grant-maintained school in south-west London, is this: an Argos catalogue, microchips and a floppy disc, a video cassette of life at school, a baseball, the diaries of Adrian Mole, essays predicting what life will be like in 2093, and a selection of popular magazines, ranging from Viz to Private Eye.
The school is one of a growing number to bury some of their treasured - and not so treasured - possessions as a sort of postcard to the future.
The pupils, who buried the time capsule last October under a statue of Athena, hope it will remain untouched for at least 100 years.
Schoolchildren in Britain and Ireland are now being invited to take part in a major project which, over the next three months, is expected to see hundreds more time capsules being made and hidden. The project, known as 20:20 Vision, will be launched by English Nature, a Government agency which exists to promote conservation of the natural environment, this month and will culminate in simultaneous burial ceremonies on February 1.
English Nature aims to raise schoolchildren's awareness of environmental issues.The project is part of the UK's contribution to European Nature Conservation Year.
"We believe this is the perfect vehicle with which to encourage children to value their local environment," said John Lincoln, an English Nature spokesman.
More than 10,000 schools and youth associations have been invited to participate. The project, which meets the criteria of key stages 1 and 2 of the national curriculum, is only open to primary-aged children in England, but to all ages in the rest of the UK and the Republic.
Video films, tape recordings, sculptures and poster displays can all be used by children to express their thoughts on the state of today's environment and their predictions for the future.
The whereabouts and contents of capsules created during the 20:20 Vision project will be catalogued at the British Museum's national time capsule databank. If things go according to plan many of the children who contributed to the project will be able to open their capsules and compare their predictions with what life in 2020 is really like.
The most famous, and ambitious capsule, called the Crypt of Civilisation, was sealed into the basement of Oglethorpe University, in Atlanta, Georgia in 1940. Its creators optimistically expect the capsule to survive until 8131 when it is to be opened.
Schools interested in taking part in 20:20 Vision should contact ENCY '95, Campaign office, Northminster House, Peterborough PE1 1UA. Phone: 01733 318351.