Personally, my favorite hobby is water bottle rocketry. You build rockets out of PET bottles, and launch them with pressurized air and water. At the time, I was somewhat of a space fan, and wanted to get into model rocketry, but never really had the money. Then, in the eighth grade, I had a teacher who really enjoyed building water rockets, so as a project he wanted us to build our own. Then, around a week later, we launched them all, and I was hooked.
So what's your hobby, and how did you discover it?