I bet Blue Peter didn't show you how to make dowsing rods out of two wire coathangers and a couple of cases from defunct biros or felt tip pens.
Use snips to cut the coathangers at the places shown by the red lines in the picture, and throw away the pieces with the hooks on, and the dead refills from the biros.
Straighten the short arms of the other two pieces to a right angle, and insert them where the refills would go in the biro cases. They should be a loose fit.
To dowse, hold one biro case in each hand, with the coathanger wire sticking out at the top with the long arm pointing away horizontally from you. It will swing around wildly at the least movement. Adjust the angle you are holding it at so that it is pointing down very slightly while you are standing still, then walk slowly forward, keeping.
The theory is that the horizontal wires will swing in towards each other if you cross running water, electric cables, or (so it is said) whatever it is that you are looking for. In practice they either stay stubbornly parallel, or they swing around with each step for no obvious reason. It may help if you can relax and walk calmly.
I did find that it always twitched more at a certain point in my livingroom (though I did know in advance that two conduits with elecric cables in them crossed in the cellar at that point) and it showed where a buried black plastic water main entered a cottage in the country. Again it was logical place for the water main to enter, so I may have expected it to be there.
It is a good way to get a group of usually sensible adults to behave like schoolchildren.