Most encyclopaedias offer similar features. The basic information comes as text, but often diversifies into a bewildering array of photographs, diagrams, films, animations, sound clips, tables, and maps. An atlas and a dictionary are often thrown in for good measure. You'll usually find research software to help with essays and projects, and updates via the Internet.
Eyewitness Children's Encyclopedia (Dorling Kindersley, age 7-11, pounds 29.99) is a gentle and colourful introduction to the reference tool, covering culture, nature, history, science and geography in a fair, but not daunting, degree of depth.
It's very child-friendly: there's a "Grab-a-Gag" section, for example, all guaranteed corny. The voice synthesiser reads back any piece of text, including your own notes. It is rather odd-sounding, but is useful for children whose thirst for knowledge exceeds their reading level.
Slightly more sophisticated, but just as easy to use is The Kids Multimedia Encyclopaedia (GSP, 8-13, pounds 19.95); its pleasing balance between clarity and thoroughness makes it an ideal homework tool. But best of all, the publishers seem really to have considered what interests children. There are articles on scouts, fairs and the former pop group Take That, for example, as well as more "serious" brain fodder.
If you really want to give your children's grey matter a work-out, however, the specialised EyeWitness Encyclopaedias (Dorling Kindersley, 10+, pounds 29.99) provide a rigorous depth of information. Science 2.0 covers maths, physics, chemistry and life science with baffling thoroughness. The matter explorer lets you magnify objects up to 30 million million times. Josh, who is eight, particularly liked the videos: "They make things way easier to understand."
Space and the Universe deals with all things cosmological, with a healthy dose of astrophysics thrown in. You can even try your hand at landing a moon shuttle, or building and launching your own rocket.
Nature 2.0 stays closer to home, with a wealth of information on fauna, flora, fish and fowl. Marvel at the complexity of species classifications, and learn the difference between an edentate, a monotreme and a pangolin.
When it comes to the big stuff, Encyclopedia Britannica has long been the gold standard of reference books; its new CD-Rom version (all ages, pounds 125), containing 45 million words and 3,000 articles, aims for the same in multimedia. It is certainly the most thorough and wide-ranging of the bunch and, once you are familiar with the rather confusing titles, it is actually quite easy to use. You can search, for instance, using whole sentences: I challenged it with "Who was Andrei Tarkovsky?" and was instantly offered a sizeable resume of the life and works of my favourite film-maker. However, when Zach, aged three, tried "What are Smurfs?", he was rather disappointed to find that the giant oracle hadn't got a clue about them.
However, when it comes to sound and vision, multimedia veteran Encarta 99 (Microsoft, all ages, pounds 49.99) has the edge. Unfortunately, like others it has somewhat outgrown its medium, having now sprawled on to two CD- Roms. This entails much irritating disc-swapping. Nevertheless, the multimedia features are particularly impressive. Among other things, you can take a virtual tour of Paris or the Tikal Mayan ruins, analyse your diet, experiment with fractals, or familiarise yourself with phrases in 60 different languages.
IBM World Book (13+, pounds 79.99) has all the usual information, but somehow it rather plays second fiddle to the added features. For instance, you can use IBM's speech recognition software, ViaVoice, to verbally navigate your way around. The Homework Wizard, however, looks particularly useful, helping children write essays, draw up charts, graphs or website pages and even creating your own timelines and quizzes - although quite why you would want to do the latter, I'm not too sure. There is also a handy virtual highlighter pen and sticky notes with which you can mark text and add comments of your own.
Oxford Interactive Encyclopedia (Learning Company, all ages, pounds 29.99) is a home-grown antidote to the US-centricity of some of the larger titles, loudly proclaiming its heritage. It's rather textual - besides the articles, there's an extensive dictionary, thesaurus and biographical dictionary - but there are some nice touches. Flan, six, was particularly taken with the planetarium; type in your location and time zone, and it illustrates the night sky from your bedroom window. "It's really good," he exclaimed, "because whenever I look out of my window it's always cloudy."