I don't care what it is that I invent, as long as I can spot its commercial potential.
When I was 14 years old, I removed the clockwork motor from an old toy and connected it to an electric motor from another toy, so that it worked 'in reverse' (i.e. as a generator). I fed the AC electrical current into into a rectifier circuit, to convert it to DC. I then used this DC power to run a small amplifier which took its signal from a radio oscillator circuit. The output was fed to a small speaker. The result was a clockwork radio. While I was pleased with my invention, I couldn't see any real use for it.
Many years later, Trevor Baylis (re-)invented the clockwork radio, saw its potential for use in the Third World, and made a large fortune out of it!
Chris