Answer:
If the Northern Hemisphere is tilted toward the Sun, it's summer, while it's winter in the Southern Hemisphere (tilted away from the sun).
Step-by-step explanation:
The Earth, like several other planets, rotates on its axis as it revolves around the sun. Moreover, the Earth is slightly tilted on its axis by 23.5°. This angle does not change as the Earth moves around the sun. This means that, for some part of the year, the Northern Hemisphere is closer to the sun, while for another part, it is the Southern one. The combination of longer days and more proximity to the sun causes days to be warmer, leading to summer.