Step 1: Material
2 old pens (optionally equals)
1 old 9V battery
1 LED (of any colour)
2 pieces of a single-line wire of 1.5mm (5cm long)
1 piece of a flexible wire of 1.5mm
Step 2: Prepare 2 Pairs of Cells
Step 3: Assembling Process
With the 2 pairs of cells prepared, stick them inside the pen's casings, keeping their tops out, in order to allow to solder the flexible wire and the LED, matching the positive top of a pair of cells with the Anode of the LED and the negative top of the other cells with the Cathode of the LED.
Solder it all together, like it's shown in the picture.
Step 4: Final Product
If the LED lights on, there's continuity from one peak to the other...