[ WP ] Explain a piece of modern technology to a person from around 100-500 CE .

Not exactly modern, but... 
 
 
 I knew who I was looking for as I walked through the Roman streets, and found it sitting in the shadow off of a large main square. A man in rags, squinting at the people around him. I knew enough Italian, very close to the Latin of the time, to find out that he was a beggar, and could n't work any more. 
 
 
 I knelt down by him. `` I can help you.'' I pulled the thin case out of my pockets, and quickly put this modern miracle on him. 
 
 
 He was awestruck. `` I can see again! I can see! How does this work?'' 
 
 
 `` The glass is curved and made to a certain thickness. It compensates for your vision problems by affecting how your eyes view light and depth.'' 
 
 
 One-third of people had vision problems. After this invention had been copied by the Romans and was widely distributed, I went back. Now, there were no Dark Ages, and the Renaissance came centuries earlier. It was as I thought. Just fixing this one thing created so many more productive people.
