Why is there summer?

I was just watching a lecture by Dr. Lawrence Krauss on YouTube about "Science & Religion" when he showed some numbers from a survey on Science Literacy.

• 53% of American adults were unaware that the last dinosaurs died before the first human arose
• 50% of adults knew that the earth orbits the sun, and takes a year to do it
• 53% of adults knew that: "Human beings, as we know them today, developed from earlier species of animals"

This was ten years ago, not in the dark ages of scientific repression of the Early Middle Ages. But it was in America, I thought, they are just dumber then we are here in good old Europe.

But then it stroke me like lightening, I recalled one question from my back then 9 years old daughter:

Why is there summer and winter?

I didn't know the answer and it astonished me. Me who holds up science as the liberator from the ages of superstition and burning of witches, didn't even know such a basic fact. I didn't have my extended brain, a computer with internet connection, near me so I told her that I didn't know that yet, but I would like her to reason with me about that; she agreed.

The first idea was that the earth was wobbling on the axis. But soon I remembered my physics teacher in Germany who answered the question of "Why does the earth rotate?" with "There is no one (no force) who would stop it from doing so.". So I knew that there is no one who would push the earth back when it would wobble to one site. That way this idea was from the table.

Then I recalled that every table globe I knew was tilted in the same angle and that there is a reason for that. The earth axis is tilted by some 20 degrees or something. Now I knew I was on the right track, so we took some random stuff, I think it was two coffee cups, to simulate our simplified version of our solar system.

She did hold the "sun coffee cup" in the middle and I rotated the "earth coffee cup" tilted to some degree on the right side of the sun. That way we were able to see that the most light would reach the northern half of our earth cup.

Then I traveled with the earth cup, still rotating to get day and night, and in the same angle, because there is no one to push the earth to another angle or twist it, to the left side our our sun cup and there it was, the answer! Now the southern half of our earth cup would get the most of the suns light radiation while the northern half would only get a little. There it was, summer and winter.

That was convincing, even for her, but then, why is the earth tilted and not straight 90° as you would expect it? That is a whole another question, and as you might expect I didn't know the answer either.