As someone who doesn’t know a lot about building electronics, I love learning about products that manage to teach the basics of the field. Bare Conductive’s Paint Pen is one such product. It’s a liquid wire that you can use to make simple circuits or electronic prototypes.
The ink also works as a solder or adhesive, meaning it can be used to repair circuit boards or attach parts to electronics. Adafruit Industries made a short video that provides a few applications of the pen:
You can buy the pen from ThinkGeek, Amazon, Adafruit Industries or from Bare Conductive itself. Price starts at $10 (USD). MIT also has a great article on conductive paint that also has links to instructions for making your own paint.