Why is the sky blue: For a 3-year old My nephew asked me yesterday why the sky was blue. I tried to explain it to him as best and as dumbly I could, but I failed. I tried to explain the concept of scattering of light using an analogy of colliding marbles, but I wasn't really successful. Can someone give this to me in a way suitable for explaining to my inquisitive 3-year old nephew?
 A: I would keep any explanation to a three year old in wholly immediate, phenomenological terms; let him do the following experiment.
As you know, the blueness of the sky's orb is a mostly scattering rather than transmission phenomenon, but this is probably too much detail. I think you need to keep the explanation along the lines of "stuff changes the light that passes through or off it" and do it with something like the following. 
Get hold of several different colored cellophanes or other transparent, colored plastics and a torchlight (flashlight or candle). A brilliant white modern LED flashlight will make the experiment more vivid. Show him how matter colors light as it is travels through matter. White without the medium present, colored with. 
Something I recall being fascinated by at that small age was blood red of a torchlight shone through fingers, particularly the webbed skin between the fingers when one cups one's hand over a powerful torchlight (this works beautifully for a child's delicate fingers). This is the same phenomenon as the cellophane coloring the light.
He should be able to then accept that the air is a very "thin" kind of stuff, but there's a great deal of it. Tell him how high he would need to walk upwards to get through the air (in terms of walking to the house of someone whom he kens well and is 10 or 20 kilometers away). So, like the cellophane, you expect it to change the light that passes through it. 
The color of the sky is the change that the stuff of the sky works on the light passing through or bouncing off it, just like the cellophane.
This approach also shows him that the phenomenon is no more mysterious - but also importantly that it is no less mysterious - than why anything around him is colored. The sky is not special or complicated in any way that his colored toys or dolls are not. The mystery is all around him, and it may encourage him to think about what color itself means and not take it for granted.
A: You'd explain it the same way you'd explain it to a fifty year old person.
What about this...
"As you know, there are different colours  ... red, green and so on. The air likes to bounce around these colours.  Amazingly enough, the air most likes to bounce around blue.  The other colours don't get bounced around as much ... they just go away.  So when you look up at the sky, we see all the blue being bounced around. The air is bouncing the blue around!"
(Older children may then ask, "Awesome! But why does the air bounce around blue and not some other colour?" The best answer then is something like, "It's because of the different size and speed of the different colours. It turns out that air is best at bouncing around blue. The others just pass through - they go away.  Look up at the sky, and you see all the blue bouncing around!")
Further, for three year olds, there's nothing at all wrong with introducing the colours as "little balls"1 within this explanation, if desired.
Looking to the other superb answers, also talk about how big the air is.  (It's possible that three year olds know what the "space station" or "satellites" are these days, with endless Netflix and iPad. Using the techniques in the other answers, you could show how much air there is, "all the way to the space stations!" "where it is black" because there is "no air to bounce the colours around" up there.)
Secondly from the other answers, you can make the excellent point about sunset. "When you're looking more at the sun, at sunset, the red balls make it through the air to you - the air bounces away the blue balls."  (Of course, this is getting less and less precise - let us say more and more 19th century - by and by.)
So that's how you explain the sky being blue to anyone, including three year olds. You explain it by explaining how it is. The air bounces blue around. But first!...

But first!!!!
Critically, as a pedagogical matter I would preface the whole explanation like this: this is more important than the actual explanation.
"That is a REALLY HARD question. It's important that you know, nobody had a clue about this until very recently. Like, when your grandmother {whatever} was alive. There was this great man called Einstein who figured it out {I'm not totally sure if three year olds know from TV the cliché of "Einstein" as our great scientist - if so, bingo, that's good enough}. Before that it was really a mystery. It's still a mystery."
I'm not quite sure of the best way to express it to a three year old, but it is critical to impress on the three year old, in terms they can understand, that

*

*humans presently know almost nothing


*it's only since the Enlightenment - very recently - that we have tried to figure things out. (Give a quick synopsis of the Enlightenment, starting with the renaissance in Italy)


*don't forget we only had electricity, phones, visited the moon, etc very recently ("you're grandmother got them")


*many many questions you can ask are very very difficult - your Mom doesn't know the answer, even your Aunty who works at the Big School with the 30 km tunnel underground doesn't know the answer


*when you're a little older, you, too, can start trying to figure out these questions, along with Mr Einstein, your Aunty etc
This "little speech" will dwarf in life significance any explanation of why the sky is blue. (BTW asking "why the sky is blue" is astounding for a three year old, I'm sure the kid is almost four!)
Trials
Unfortunately I don't have a three year old on hand at the moment - that is awfully young - but testing shows this is perfectly understandable to 7, 8 year olds.
The bottom line three year old explanation is: "air likes to bounce around the different colors.  But mostly it likes to bounce around blue.  So when you look up at the air, you see all this blue."
(The other colours "pass through" or "go away" ... "pass through" is too complicated a concept for three year olds, so wing it there.)

1 given that currently, our base understanding of the fundamental nature of energy and matter is: Utterly Nothing Whatsoever: there's nothing wrong at all with talking about "little balls!" to a THREE year old!
A: None better than a local university can answer this question with their statement:

A clear cloudless day-time sky is blue because molecules in the air scatter blue light  from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight.

Source: http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html
I think the only way to make it clearer for a young individual that has no concept of molecules would be to explain as follows:
In the day time, the sky is blue because small balls of energy move blue light more than other colours.
This may also need the explanation that those small balls of energy (a simple way of saying a particle or baryonic matter) exist at all times. That is only if you decide to use that way of explaining it, but I'm sure you'll adjust it. Nonetheless, I hope this helps you.
A: Based on the first 2 answers, I tried to kill 2 birds with 1 stone:
I capitalised on my explanation to give him a life lesson going forward, as he is sometimes bullied in kindergarten.
I showed him a variation of the filter experiment suggested, and then went as such:
Imagine the dust particles of the air to be bullies (as scattering agents) to the little light particles (just like that, I told him they were called photons). We can say that blue light is more determined to achieve success in the face of bullying (more energetic) while red light is less so.
Then, with time, the red light can show their achievements either early or late, when they haven't been bullied as much, out the blue light has been bullied (scattered) too much (the sky is red at dawn and dusk), but the blue light is determined to keep going on, and stands up in the face of much bullying and their achievements can be seen as long as light is there other than when it is red (the sky is blue during the day).
I told him finally that he should strive to be like the blue light who succeeds in the face of adversity. As such, he seemed to accept it.
I know that this is not theoretically appropriate to any extent, but please forgive any transgressions on my part. I also told him that I would try to explain it more literally to him as he got older and learned more of Physics, which he now seems to be very interested in.
I'm looking forward to posting a few of his next questions. Thank you.
