If you went back into time, say 1000 years, how useful would you be to people then? Being able to do C++ programming won't matter much if you don't know your legumes from your weeds and starve to death....
Would you be able to "invent" electricity? Radio? Could you explain how to till a field? Or what cross-pollination was? How about just washing your hands? (And with the non-existent soap?)
Simple quiz... I only got 8/10 right. I'd be able to bring people to the 19th Century. Maybe....