One reason to explain we have seasons is because when it's summer
(hot), the sun is shining on the northren hemisphere. When it's winter
(cold), the sun is shining on the southren hemisphere. Its also because
the Earth is tilted at a 23.5 degree angle on it's axis. So when the Earth orbits the
sun at an angle, it is closer to the sun at different points. That's what causes
different seasons.