Saw this question on a Web site, and I thought it was pretty interesting:
I am looking for items that match all of the following conditions, and I'd love some help, if any of you would be kind enough to provide. Which items (products or processes) satisfy all these criteria:
They were unknown to people in ancient Rome circa 150 B.C.
1. They could be manufactured with then-existing technology and then-available raw materials.
2. They would be at least modestly useful in that era.
3. Even a nontechnically minded person today -- say, a smart 12-year-old -- would know how to make and use them.
4. Their absence would be pretty clearly visible.
One example is the stirrup: It apparently wasn't introduced into Europe until many centuries later; it's not hard to make; it can be quite useful for riders, and very useful for mounted warriors; anyone who knows they're possible and useful can know more or less how to make them; and if you see a rider without stirrups, you'll probably notice something wrong. I have heard it said (though I am less certain of this) that whipped cream was also unknown in Rome, but I'm not sure whether efficient whisks could be easily made using the metallurgy of that era. The technique of using cowpox as vaccination against smallpox also qualifies, since many laypeople today are aware of it, and aware that it was invented only several centuries ago
In any event, you get the point. As I mentioned, I need something that satisfies all the conditions. If it just satisfies most of them, that's not enough for me. If the accuracy of the answer isn't obvious -- for instance, if it's surprising that the Romans didn't have this -- please also pass along some sources establishing the accuracy, if you have them. Thanks very much!
UPDATE: Forgot to include (1) arabic numbers (actually, my favorite example), (2) sterile technique for surgery and wounds, and (3) distillation, the basic principles of which many, though probably not most, people know.
People suggested lenses (for eyeglasses, telescopes, and more), but I understand that the ancient Romans of 150 B.C. did not have clear glass -- am I mistaken? Gunpowder would not, I think, qualify; most people don't know the ingredients, and even if they know it's charcoal, sulfur, and saltpeter, they don't know what saltpeter is. (Don't tell me what it is -- the important point is that most people wouldn't know it.)
Toilet paper.
Rigid horse collar.
Moldboard plow/harrow.
There is some question as to whether or not the Romans had nailed horseshoes at that point in time.
Believe it or not, they didn't have the tomato. It wasn't known in Europe until 1522, when Cortez' men brought it over from Mexico.
I know this has nothing to do with the questions, but when you mentioned whipped cream, it came to mind.
Wheelbarrow.
Spaces between written words (facilitates silent reading).