There's no foolproof way to prove that your sister actually stole your sunglasses, but polygraphs can sometimes do a pretty good job. Here's how to throw together a DIY polygraph machine with nothing but an Arduino, some wire and some tin foil.
While we think giving someone a lie detector test is a little over the top, it's a very cool project to test out the Arduino's strengths (not to mention scare your more deceitful friends). And, you don't need a ton of electronics experience to get it working -- if you've checked out our guide to creating Arduino gadgets with other people's code, you're probably ahead of the game. It's actually a galvanic skin response device, not a real polygraph test -- but it should do a good job of measuring their sweat levels through their fingers, which is one of the many things a real polygraph actually tests for. Hit the link to see how it's done, and if you're planning on building one, you might want to check out how to beat a polygraph test so your project doesn't backfire on you.