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

You'd think that our modern world would be overtly impressive to a time-misplaced Roman. This would have been the case with some but Quintus had a remarkably good understanding of the modern world. 
 
 Even my central heating was comprehensible -- although he suggested that I look into getting a system that heated the floor instead of blowing warm air through vents as `` Heat rises of its own accord.'' In the end the only things Quintus would find truly alien were my electronics. 
 
 When he discovered my garage. I heard an excited `` Oh Fortuna!'' and hurried after him. Quintus was looking at my metal lathe. The time-lost Roman' was giving the machine the geek equivalent of bedroom eyes. `` This, this I recognize. You turn things with it, cutting them into round shafts.'' 
 
 His gaze shifted to the floor. `` And this stone, this is n't stone. This is caementium -- polished smooth! And I see this is where you leave your carriage, no doubt to protect it from thieves. And you even have plumbing, truly this is a wonderful land! 
 
 `` No doubt you have questions about how the carriage moves?'' I asked. 
 
 `` My dear friend'' replied Quintus, sounding terse and annoyed. `` It obviously runs on some sort of Aeliopile. I've read of them in Vitruvius's de Architectura and did much engineering at Hieropolis. I am, if nothing else, a learned man. Might I borrow your lathe? Afterwards, we ought to use that hot-tub -- I do so miss the baths. Do you have a slave handy for the rubbing of shoulders and aching joints?''
