During the day, the sun shines brightly and heats up the sand in the desert, making it very hot to touch. But at night, when the sun goes away, there’s no warmth left, so the desert becomes very cold. This is why deserts can be hot during the day and chilly when the stars come out!
Caption for your book: "The desert sand sizzles under the sun's warm rays, but at night, without the sun, it feels as chilly as a winter's breeze!"