Ravi tests which liquids carry electricity with a battery, a bulb and two wires

Why

Ravi is repeating something almost everyone believes, and it is nearly true in daily life: the water we actually meet, from taps, rivers, our own skin, does conduct, which is why water and electricity are a dangerous pair. But it is not the water molecules doing it. Pure water conducts very poorly; what carries the current is the salts dissolved in it, split into charged particles that drift and ferry the charge across. Tap water has picked up calcium and magnesium salts from the ground and the pipes; distilled water has had them taken out. Test: stir a pinch of common salt into the distilled water and the bulb lights at once, same water, ions now present. Neeta is right: the tester works, and it is the dissolved salts, not the water itself, that conduct.