Glass ones don’t. Is it just a weird design quirk or does it actually serve some purpose?
I think the folding gives the plastic rigidity where required in that area. Glass is strong enough without.
That’s an excellent assumption. I don’t know the answer - but my first instinct was PROFITS! PROFITS! PROFITS! They make them like that so they can sell you less soda for the same price! It’s a conspiracy!
But then I realized that it’s, what, 1/2 an ounce of sugar and water. Hardly enough to cut into their markup.
Oh well, back to making my tin foil hat.
WAG here: I’m guessing that you’re young enough not to remember (or maybe you just never noticed) that 2-L soda bottles used to be made of two pieces: the clear plastic bottle, which had a perfectly hemispherical base, and a separate opaque plastic piece that was glued to the bottom of the bottle. I believe I heard at some point that the change between the two was occasioned by improved blow-molding techniques, but I might be misremembering — for all I know, it was just something that nobody had ever thought of before.
What gives me some confidence is that I remember that when plastic bottles first came out (at least here in Australia) the bottom of the bottle itself was a simple rounded end, of much the same thickness and strength as the sides. Then there was a separate thicker plastic part, like a cup, that provided the necessary toughness and stability and rigidity, bonded to the bottom of the bottle itself. It was (at least here) usually black.
Then (I’m assuming) they figured out how to make the bottom rigid and strong enough to survive on its own so that they could use a one piece design, thus reducing production costs.
Princhester (who’s childish hobbies include making water rockets out of bottles and thus knows far, far more about plastic bottle shapes than any sensible adult should).
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I’m certain that it is to give the bottle rigidity and the ability to stand up, as has been stated. I don’t know if it’s improved blow-mold technique (I would’ve thought they’d had that worked out long before) or them just getting cleverer about it. There is a desire for profit – but it’s not the need to save the small amount of soda – the amount hasn;t changed, it’s marked on the bottle. But, as someone has pointed out, the older version (here in the States as well) had a two-piece construction of a round-ended plastic bottle stuck into a base. It costs money to mold a separate base (out of different plastic, too – the bottle’s PET, and I think the base was Polyethylene) and go to the expense and trouble of gluing them together. Making it all one piece of the same plastic saves a lot of time, effort, and materials.
Possibly the reason they didn’t do it earlier was the need to keep the contents under pressure. The old round-ended bottles used the classic rounded design that gives great strength against internal pressure (if they don’t keep pressure, your soda goes flat). If you just molded squared-off end on the bottle, as with a glass bottle, the stresses woulf concentrate near those bends and the bottle would crack when it was being handled. The current five-point design uses less -tigthly turned corners and distributes the stresses so that the botles won’t break. I’ll bet it took some time to figure that out.
And would have been lighter and more aerodynamic. Bummer that I started making water rockets after they stopped making these bottles.
The old one was harder to recycle too.
FYI and of no real help, current bottles are made from injection molded blanks that look like a big thick fat test tube with a threaded end for the cap. These are then blow molded into the finished bottle shape.
These two-part bottles are still in use in some places. Eastern Europe, for example.
Actually, in many places they are refilled rather than recycled. When I lived in Budapest in 2003, you paid a deposit on 2- and 3-litre Coke bottles, which you got back when you returned them to be refilled. I can’t say I was impressed with this arrangement, since the cola always had an off flavour; I presume this had something to do with either the bottle material or the cleaning process. The non-refillable plastic bottles (1 litre and less) did not have this problem.
Yeah, but why “5”? Why not 3, 4, 6, or 7?
J.
I actually wrote to the container-design company Ball about this (you know, the folks who used to make canning jars and now do lots of plastic containers). I’ll let you know if they write back! (Forgive me for slightly misrepresenting fellow Dopers as my “students”, but I thought that that would make it more likely to come across as a serious, if kind of peculiar, inquiry rather than a silly prank.)
My guess: the same reason office chairs anymore have five caster legs – it gives them a lot of stability and harder to tip over. A three-legged chair is very stable in that three points determine a plane, but you don’t have to tip it up very far (especially if the legs ar on wheels) to tip it over. Four legged things tend to get aligned with the pairs of legs parallel to the way you’re sitting. But with five legs, if you lean backwards, you’re going to be pushing toward one unpaired leg, and it’ll be harder to tip.
So with bottles, I think three or four nubs don’t give you enough resistance against easy tipping. You could go higher, but the more nubs, the tighter the curvature becomes, and the more likely the bottle is to split. Five is probably a good compromise.
The answer I got from the folks at Ball, although not very detailed, seems to bear out Cal’s reasoning:
Boy, that raises more questions than it answers, doesn’t it? “To help in shelf life”? How would a five-footed bumpy bottle bottom prolong the shelf life of the bottle contents, I wonder?
My guess (these are all guesses on my part) is that he’s referring to the total design “retaining freshness”, not just the number of legs. As i said earlier, the bottle not only has to stand upright, it has to retain pressure as well, or else the soda will go flat. That requires relatively gentle curves, since tight curves tend to concentrate strain, and if it’s too tight the bottle will break. Five feet is a good enough compromise for stability (the bottle doesn’t fall over easily, which is good if it’s open) , yet will keep its integreity when the contents are under pressure.
Those nubs are where the Mentos fit. If you look down the bottle after you drop one in you’ll see.
You can see it better if you look directly down into the bottle after dropping the Mento(or better yet, a few of them) in it. 
You have a link on making those?
There are zillions. Just google water bottle rocket and you’ll find more than I could ever give you.