Miles stands mesmerised as with a hiss and a whoosh a full-sized model of a man, suspended from thick ropes high overhead, is rudely dumped into a perspex bath.

As the water splashes over the sides, carefully stage-managed not to soak anyone, Miles announces: “Eureka! - that’s Archimedes. He discovered that when you get into a bath full of water it would splash over the sides. He was so excited he ran into the streets shouting ‘eureka’, forgetting he was naked. ” Well, almost.

At Eureka!, the UK’s biggest hands-on science museum for children, visitors are treated to the spectacle of Archimedes taking a bath in the foyer twice an hour.

Since the Halifax-based museum opened five years ago, throngs of wide-eyed children have been inventing, creating and communicating with wild abandon. There are no “do not” signs, no roped-off areas or glass showcases. All 450 exhibits are there to be explored, played with and learned from.

Young visitors can step inside a giant mouth to find a wobbly tooth, edit the front page of a newspaper or run their hands over a massive tongue, learning which bits taste what.

Earlier this year Eureka! changed its image slightly - it acquired a roped-off area. It is not to keep children out but to keep them in while they role-play as jungle explorers. Spotting a gap in the market for museum education provision for pre-schoolers and under-fives, Eureka! developed its Jungle Explorers workshop and teachers’ pack.

Liz Smallman, the museum’s education programmes co-ordinator, believes this will scotch the myth among nursery teachers that the museum is only for older children. “We realised we had nothing structured for pre-school groups, so we spoke to early years teachers and playgroup leaders,” said Ms Smallman. Ideas included providing clean clothes for children who mess up their own and role-playing through workshops.

“Jungle Explorers expands our educational opportunities for under-fives and the teachers’ pack considers the practical support and back-up facilities required with small children,” she says.

The emphasis is not on reading - “We want to get the language going first ” - and all exhibits are geared to language and maths skills at key stages 1 and 2, says Ms Smallman. Teachers can make preliminary visits.

Jungle Explorers is filled with musical flowers, large animal-shaped jigsaws, a big gorilla to sit on, a tree house with secret trap doors that exude nice smells, a ball pond full of hidden fishes. The entrance to the Jungle Explorers workshop is shaped like a giraffe. Children pause to tickle its tummy as they pass through.

Sarah, one of six specially trained early-years helpers, constantly encourages children to crawl under and through the exhibits and explore smelly cubby holes built into the tree house.

Teachers and adult helpers get a task card and pack, which they are urged to photocopy for parents, who are welcome to be present. There is an expected ratio of one adult to four children. Ms Smallman would like it to be one to one.

In the music area children can create their own sounds and rain forest noises. The workshops last one hour - 30 minutes in the jungle and 30 minutes in a well-equipped classroom making animal masks with musical accompaniment from Walt Disney’s Jungle Book.

The level of hands-on activity pioneered by Eureka! has been designed from conception. For other museums such an approach could destroy most of their exhibits.

The London Transport Museum in Covent Garden, which is packed with irreplaceable vehicles and artefacts, takes a more traditional, albeit equally entertaining approach to its under-fives programme.

Every Wednesday and Friday during term time and five times on Saturdays, Ruby Rover, resplendent in her jester-like outfit, bustles through the entrance of London Transport’s museum. She makes her way to the brightly coloured omnibus parked in one corner, snatching frequent glances at her watch while keeping an eye on the visitors coming in.

As soon as a young child appears, she grins widely, beckoning. “Hello, my name is Ruby Rover, what’s yours? I love to travel. Would you like to come with me on a voyage of discovery?” The accompanying adults are invited too.

As soon as Ruby manages to coax enough under-fives to join her, she yells, “All aboard now” and whisks them off on a tour of the clanking, wheezing heritage of trams and trolley-buses with the promise that she has special permission to take them on buses that are closed to all but this privileged group.

These 30-minute programmes, called Platform Promenade, make up one of two initiatives launched last year by the London Transport Museum to fill its gap in museum provision for nursery and KS1 children. The other, Time Bus, concentrates more on Years 1 and 2 and involves more gallery investigation.

Stephen Allen, London Transport Museum’s education services manager, concedes that museums have ignored this age range. “We found there was a disproportion between what we provided and the numbers of under-fives regularly visiting us,” he says.

“Before the museum was relaunched in 1993, the curriculum was geared to including transport at KS2 with a specific topic on land transport, so we had a captive audience. In the Dearing review land transport was subsumed into two study units on Victorian transport since the 1930s. We realised we had to work harder to attract groups. Also we were finding that, while doing very little for KS1 groups we were still getting visits from them and nursery groups. ”

The museum also realised that nursery and KS1 teachers were not as confident as other teachers in using museum facilities, and not as familiar with museum education departments.A schools officer was brought in to collaborate with teachers to create programmes. It included preparation meetings with teachers and helpers to structure visits and decide on a script for the performances.

Since the programmes began there has been a 20 per cent increase in the visits from nursery and KS1 classes, which now make up about 57 per cent of visits.

The Platform Promenade activities are worksheet-free so the children can concentrate on the vehicles while absorbing the sensation of travel through shapes, sound, storytelling and puzzles. Although tightly scripted, they are adaptable.

“For most under-fives it is enough just to know they are going on a trip that involves a train journey. To find lots of exciting things to see, touch and explore as well is a bonus,” says Jan Jones, who took a group of three and four-year-olds from Aulton Primary school, Leeds to the jungle at Eureka!

Eureka!, The Museum for Children, Discovery Road, Halifax HX1 2NE. Tel: 01422 330069. Workshops cost from Pounds 1.5-Pounds 3 a child. Minimum of 10 children, one adult free with every four children. The price also includes entrance to the rest of the museum.

* London Transport Museum, School Visits Service, Covent Garden Piazza, London WC2E 7BB. Tel: 0171 379 6344. Pounds 2 child. Minimum of eight children, one adult free.

Additional adults Pounds 2 and free teacher preparation meetings