The other day I wrote about a knife switch that might actually be an antique.
Today, allow me to present a knife that could be 2000 years old!

It's not, though. It was probably beaten out of a bit of old leaf spring or something by some bloke in China only a few months ago.

It's about 235mm (9.25 inches) long when open, and cost me $AU11.50 delivered (about $US10.50, as I write this) from eBay dealer "The Medieval Shoppe" here in Australia. Here they are on ebay.com; here's a search that finds this "Rustic Foldable Iron Knife" in their ebay.com.au store (if they currently have any to sell), and here it is on ebay.com.
The wood of the handle was a bit rough and splintery, so I sanded it a little and applied some home-made beeswax polish.

The blade came with a usable edge on it. I straightened the edge a bit on a steel, then touched it up with my fancy sharpening doodads. It took longer than usual to remove a tiny amount of material, so the blade is probably pretty high-carbon steel. I wouldn't be surprised if there's a lot of variation in the steel used to make these things, though.
(I cordially invite readers to start a religious war in the comments about What The One True Knife-Sharpening System is. I'll start: If you're a beginner and/or clumsy, get a sharpening kit with some sort of angle guide, like the CRKT Slide Sharp or, for blunter blades that need more material removed, a Gatco sharpening kit.)
Even without extra sharpening, this slightly-mad-slasher-looking thing's not just a bit of renaissance-faire costume kit. It's a perfectly practical tool, with a nice slim blade profile that makes it good for slicing tasks, though not a great choice for really heavy cutting, and no use at all for prying things open. And if you were to hop in your time machine with it and go back to the Roman Republic, nobody would find it particularly remarkable.
Well, actually I suspect the cap on the hinge rivet may be a bit of aluminium or stainless steel, which'd be a giveaway if someone examined the knife closely. But apart from that, this is a decent ancient replica.
(You could be onto a nice little earner there, actually. Scour handle and blade with dirt for a while, soak it in wine, soak it in oil, put it in a low oven for a little while, then bury it in the garden and water it daily for a week. And then dig it up and put it on eBay with a $500 reserve as Roman Pocket Knife Miraculously Preserved In Peat Bog. Just hope they don't carbon-date the wood.)

More intelligent readers may have figured out, from this, that folding pocket knives have been around for a surprisingly long time. Fixed-blade knives are stronger (provided they have a decent-sized tang), and simpler, and so have always been much more common. But the ancient Romans did indeed have folding pocket knives - some of quite sophisticated design.
The ancient folding knives, and indeed every known folding knife until about the 15th century, don't "lock". The blade is only held open, or closed, by friction between itself and the handle. This design makes it easy for the knife to close unexpectedly and seriously injure you if you push the blade hard into something (or someone), or if you're cutting something and the blade jams on a push stroke. A simple friction-joint knife can also come open in your pocket; hilarity may ensue.
For this reason, it's now possible to get a variety of Swiss Army knives (find more with the SOSAK "SAK Selector"!) that have a proper lock for their main blade, and not just the standard "slipjoint" arrangement that resists opening or closing of the blade, but doesn't positively lock the blade in either position.
(And yes, I am aware that some law-and-order geniuses in the UK decided to one-up the similar geniuses here in Australia and make it illegal to carry any knife with a locking blade in a public place, unless you have a "good reason". The list of acceptable "good reasons" does not appear to include "not wanting to cut my own fingers off, or stab myself in the scrotum while running for a bus".)
I wouldn't be surprised if someone 2000 years ago came up with at least a simple locking mechanism for a folding knife - like the rotating collar on the classic Opinel knife, for instance. A sliding ring, a peg that goes through holes in the handle and a slot in the blade; there are lots of possibilities that wouldn't require the precision fabrication techniques and tough steels upon which modern locking knives depend. But if someone did come up with a locking folder in 100 BC, it apparently didn't catch on.
(It's possible that many such knives were made, but didn't survive to the present day. Iron and steel items of all sorts are hard to find in archaeological digs, because iron easily rusts away to nothing over time, leaving archaeologists puzzling over the stain the rust left, and whatever parts remain, to figure out what the now-lost iron parts looked like. Older bronze-bladed knives and swords often fare a lot better. Early iron blades were actually clearly inferior to the bronze alternatives; iron was much more common than the copper-and-tin used to make bronze, but until we figured out how to make proper steel, iron swords were made of wrought iron. That material makes dandy door hinges, but lousy blades.)
My replica has a neat pseudo-lock system, though:


There's a flattened spike on the back of the blade that stops it from opening too far, and is also easy to grip when you grip the handle, and thereby prevent the blade from closing. It's not a real lock, and it sticks out awkwardly when the knife is closed, but as a safety feature it's a lot better than nothing.
(The spike also has a little hole in it, through which you could tie a lanyard.)
Making your own knife, often from some cast-off piece of steel like an old file or a railroad spike, is a popular simple metalworking/blacksmithing project. I think a lot of people are put off the idea, though, by thinking they have to make something that's somewhere near modern commercial quality, or at least as good as a Douk-Douk or K55K.
You don't, though. You can make a knife like this with basic hand tools, a gas stove and the very cheapest of eBay Anvil-Shaped Objects, if you've already got a chunk of steel.
Or you can just buy one, of course. Either way, it's another very satisfying object.