A Victorian lamp holder immersed in a tank of murky water is very difficult to identify by feel in rubber gloves. I tried it in the "feely tank" at the new Water For Life gallery at Kew Bridge Steam Museum in west London. The lamp holder is included as an example of the sort of thing that used to get washed into the sewers.
The gallery makes an interesting addition to the museum, which is housed in a pumping station built in 1837. Lottery funded, it uses artefacts, photographs, diagrams and factsheets to tell the story of how London's water is piped from reservoir and river and how sanitation has developed since medieval times. The centrepiece is a replica section of Thames Water's 52-mile Ring Main, completed in 1994.
Altogether the museum has nine massive steam engines on show, all in gleaming condition, including the world's biggest working beam engine.
There are also old objects such as pipes and pumps relating to the history of water supplies.
School groups are guided round the impressive collection by the education officer, Derek Gooding, who has been involved with the museum since it opened in 1975.
A trained engineer and steam enthusiast, he can be relied on to ensure that the least technically-minded visitor will have fun as well as understanding the mechanics of pumping. Visits can be tailored to suit whichever topic a teacher nominates - history, geography, art, design and technology, science or maths.
As we stand in the Engine Room peering down into a channel from the Thames flowing murkily through a chamber beneath the floor, he says I should look out for Chinese mitten crabs. They get shipped by accident from the Far East to Tilbury Docks and then seek fresh water upstream. Mr Gooding has many such tit-bits of information.
Next, having got me to hold a tin of water, he demonstrates the workings of a small syringe. "Children are always keen to help with the squirting," he says.
He points to the huge dark green pump of the pioneering Boulton Watt beam engine dipped into the water in front of me. "Think of the beam as a see-saw with 'Superman' - the engine - at one end," he suggests. "Obviously the pump works when he lets it down."
In the same graphic way, surrounded by several massive engines in the Hall of Steam, Mr Gooding demonstrates the power of steam with a milk bottle and a balloon. Observers are left in awe of its strength and in no doubt about the momentous role it played in the Industrial Revolution.
Schools can arrange, at extra cost, to see two of the engines in steam. They also have the option of starting their visit with a short scene-setting walk beside the Thames, which is two minutes away across the road.
Browsing in the Water Gallery you soon realise just how wide ranging the museum is, especially when Mr Gooding highlights intriguing details.
For example, the window sills were invented after the Fire of London to stop flames creeping up walls. I also see how the first water pipes were hollowed out of trees - giving us the terms "branch pipes" and "trunk roads".
Kew Bridge Steam Museum, Green Dragon Lane, Brentford Middlesex. Tel: 0181 452 8567. School visits cost Pounds 60 for up to 30 students; additional students Pounds 2 (up to a maximum of three classes); engines in steam from Pounds 80 extra