NOTE: This instructable covers a harmless prank device, not a weapon or a destructive device. It utilizes toy strip or ring caps used for children's cap guns, found in many toy and grocery stores.
The "exploding pen" is a common item found in novelty and magic stores. These ugly prank devices somewhat resemble a crudely molded pen, and contain a small mousetrap-like device that sets off a toy cap when the pen's cap is removed. As a kid, I was very excited to acquire one, and quickly improved it by housing the initiating device inside a real pen. After pranking a couple of my friends with it, one of my friends was struck on the face by a tiny spark. He was not injured in any way, but this incident caused me to shelve my pen for safety reasons.
The primary design problem with the original "exploding pen" is that the detonation of the toy cap is relatively uncontained, allowing a shower of tiny and usually harmless sparks to fly in all directions. I have included a picture of my old pen firing. The odds are low, but occasionally, these sparks can connect with the target of your prank and cause minor pain or even a freak accident like this one.
My redesign has addressed these safety concerns on two fronts. First, by enclosing the toy cap and initiating device inside the body of a pen, almost all sparks are contained. Second, simple body mechanics make it difficult and unnatural to click a ballpoint pen while pointing it at your face.
I decided to modernize the design by housing it in the ubiquitous Bic Clic Stic, often available for free as a disposable advertising medium. I give you the improved, safe "exploding pen".