Picture it: you can go into the past, anytime you want, anywhere you want. You can bring back one thing to introduce to the people of the past. What is it?
This is tricky, since bringing back an iPad to give to Julius Caesar will be pretty useless, except confuse the fuck out of archaeologists (unless you want that. That could also be fun).
I would bring back the designs for the printing press to about the year 1000 or so, but still keep it in Europe. I'm pretty sure they would be able to replicate it or a reasonable facsimile of it, and literacy would boom so much faster (even if easier paper production was still a long ways away). We might lose some of those beautiful hand-written manuscripts, but it is a sacrifice I am willing to make. Where would we be today if the "population" became literate that much earlier in our history?
What would you bring back?