This is one of the cutest products that ThinkGeek has on their site: an electronic butterfly in a jar.
This little guy is on a wire that has sensors. When you tap the lid or make a sharp sound, electronics in the cap make the wire wiggle and the butterfly fly all around the inside the jar. According to ThinkGeek, it also sits on the side and flexes its wings like a real butterfly.
When I first discovered these some time ago, someone mentioned on the Twitter that they had bought one for their little geeks. (@gibbslamey, was that you?) If you did, leave a comment!
Available on ThinkGeek or if you're in a rush, Home Hardware.
See it in action: