what causes the seasons?

Let's start by saying what DOES NOT cause the seasons. Well obviously the seasons are not caused by lots of things, but there is one thing that is often falsely accused of causing them.

The seasons are not caused by the change in the distance of the Earth from the Sun during the year.

What are the most obvious things about the Sun in summer compared to winter?

1) At midday it is higher in the sky
2) It is in the sky for longer (the days are longer)
3) It feels hotter

If we can explain those three things you'll have a fair idea of why seasons happen.