i have often thought about how cool it would be to go back in time and introduce ideas or technology to various people or civilizations from the past. for example, you could back and play Beethoven's 5th symphony for J.S. Bach. he would probably be confused, but it would still be cool. you could bring a stereo with you and play it in a medieval court. you could take a car with you and teach people living before the industrial revolution how it worked. you could introduce Newtonian physics 100 years before Newton. you could give the ancient Greeks the hint to explore steam power (which they almost did). you could appear at a critical naval battle in history armed with a nuclear submarine, and wipe out one side. you could invent the internet before it existed. you could 'write' a bunch of famous songs before they were written and make tons of money!