You know those balls they have on powerlines that flash? Easy right? It's just a flashing light. That's what i always thought at least. Until someone pointed out to me that they sit on one line at a time with no earth and no battery, just a capacitor to store charge between blinks. So how do they create a circuit? There's only one wire!
Well since there's current flowing through the wire there's induction and you can wrap that wire in what is essentially half an AC transformer and create a whole new circuit at much lower voltage. In fact the blinking lights on powerlines internally just have two metal plates either side of the slot that goes over the line and clips in. From induction they leach power from the power lines despite there only being one wire and no earth in the whole system.
Which i find really cool and simple. You can take any high voltage power line and a metal plate and power a blinking light just by getting close to it!