Here is a surprise: summer does not happen because Earth gets closer to the sun. The real reason is that our planet is tilted, like a spinning top that leans to one side.

As Earth travels around the sun each year, the tilt stays pointed the same way. For part of the year, your half of the planet leans toward the sun — sunlight hits it more directly, days get longer, and you get summer. Half a year later, your half leans away, sunlight arrives at a slant, days get shorter, and winter comes.

That is also why seasons are flipped on the other side of the world. When it is snowing in New York, kids in Australia are having summer at the beach. Same planet, same sun, opposite lean.

Earth's tilt is about 23 degrees, and scientists think it came from a giant crash billions of years ago — possibly the same crash that made the Moon. So every snowball fight and every beach day traces back to one very big bump.