From http://www.reddit.com/r/programming/comments/ps4wn/youporncom_is_now_a_100_redis_site/c3s3xp4?context=3. Click the link for more questions!
Okay, see this stick? If I swing it, it moves swoosh. If I hit it against something, it stops moving smack. If I let go of it, it falls down clatter. You're not very surprised by this, right? That's because everything in the world uses the same rules. About three hundred years ago, a guy named Isaac Newton wrote down all the rules, and we call them the laws of physics.
Scientists still use his versions of the laws for all sorts of stuff, but there are a couple places they found where things are a little bit different. One of them is for things that are very, very small. So if I break this stick in half crack, both halves work just like they did before, right? swoosh, smack, clatter And if I break one of the halves snap it does too. If you had a tiny saw and a microscope, you could keep making smaller and smaller sticks, and they would all work the same way, right? Well it it turns out that when things get small enough, smaller than things that are too small to see, they start to act a little bit weird.
So imagine this stick is just one of those tiny tiny pieces inside the stick. If I throw it to you and you catch it, then someone uses a stick-finding machine, it might turn out to still be in my hand, or it might be in your hand like it would if it was a normal stick. Yeah, it's weird: scientists were really confused about this when they started seeing it, and a whole lot of them working together took about fifty years to get it right, because it's so strange. Eventually they figured out that the tiny stick, and everything else that small, is actually always in multiple places at once. So even though we think it's in my hand, it's actually also in my other hand, and already on the ground, and still in my hand but just a tiny bit to the side of where we thought it was. Even stranger, the stick is more in some of these places than it is in others: most might be in this one spot in my hand, but less in my other hand, and just a tiny bit on the ground. And it's all still the same stick. This is all really weird, but one thing about it is still perfectly normal: all of the places the stick is in still follow Newton's laws. If I drop the stick from my hand onto the stick on the ground, it'll stop when it hits the ground, and the pieces will add up, so most of the stick will be on the ground.
(I think now would be a good point to mention that yes, a real conversation with a 4 year old wouldn't go like this. You'd have to stop and answer questions and re-explain pieces of it. I'm just proof-of-concepting this.)
Okay, so remember when I said the stick could be in more than one place? It can also be going more than one speed! So some of the stick could be on the ground and not moving, and some could be in the middle of falling down, and some could be just starting to fall out of my hand so it's still moving slowly. Another very smart man, this one is named Werner Heisenberg, figured out that the more different places the stick is in, the less speeds it is moving in, and the more speeds it has, the less different places its in.
(PS: grammar Nazis, it's really "less," not "fewer." Can you figure out why?)
So if all the tiny parts of everything are acting this strange, how is everything so normal when you get back to big things like us? Well, there is one more odd thing about tiny tiny things that I haven't told you yet. Say I have TWO tiny tiny sticks. You know how one can be in a whole bunch of places at once? Well it turns out that sometimes you have to take both sticks together to figure out how much is in any place. So maybe for most of the tiny sticks, stick one is in my hand and stick two is on the ground, and there's also some where stick two is in my hand, and stick one is on the ground. But, there's no sticks where both are in my hand at all, even though both sticks by themselves are at least a little in my hand.
(This is where the kid will have the most questions, and also probably where anyone reading this is going to have questions, so ask away. Though tell me whether I'm allowed to use grown-up words like "particle" or I have to keep saying "tiny stick.")
When how much of one tiny tiny thing is in one spot depends on how much of another tiny tiny thing is in another spot, scientists call those two things "entangled." That makes it sound like it's a special, weird case, but it's really the other way around. Scientists go through lots of trouble to get tiny things that aren't entangled so they can study them, but just about everything is entangled most of the time. All the tiny tiny pieces of stick in this actual stick are very entangled with eachother. That's how big things like us and this stick don't seem like they're in more than one place at a time. The pieces of stick are all in a few places at once, but every different group of tiny sticks adds up to the big stick in my hand, even if the tiny pieces could be swapped around a little.
(As a final note, I've always heard the phrase as "if you can't explain it to a particularly bright 4 year old, you don't understand it yourself." I think we lost the dim one in the second paragraph.)
