The sun is the earth's heater. The ways to get warmer from a heater are to get closer to it, to have the heater give off more heat or the face the heater more directly. The earth is about the same distance from the sun during the year (with a minor variation), thus this does not explain the seasons on earth. The sun gives off the same amount of energy (with only a minor variation) during the course of a year. Thus, this does not explain the seasons. What does explain the seasons is how high the sun is in the sky. It is this change in sun angle during the course of the year that causes the seasons. In other words, it is how a place on earth faces the heater that determines how much heat that location will get.

Think about a flashlight. The flashlight is brighter when looking directly at it as compared to looking at it from an angle. In the same way, the sun light is more intense when the sun is closer to being directly overhead in the sky. On the daily cycle, this is why temperatures are usually warmer in the afternoon because the sun is higher in the sky. During the course of a year the sun is higher in the sky in the afternoon during the summer. In the winter the sun is not as high in the sky.

A demonstration is to place a sun in the center of the room. Have someone walk with a globe around the sun and always keep the North Pole pointed toward the same ceiling corner of the room. Have another person stand between the sun and earth. That person will notice that during part of the earth's walk around the sun that the North Pole can easily be seen and during another part of the walk the North Pole can not be seen. The North Pole being easily seen represents summer when the sun's rays are more direct. When the North Pole is tilted away from the sun this represents winter when the sun's rays are less direct.

In conclusion, the seasons are caused by the earth's tilt that changes the amount of energy a point on the earth gets as the earth revolves around the sun.