Imagine the desert as a giant sandbox. During the day, the sun shines brightly and warms up the sand. The hot sand then makes the whole desert warm and hot.
But, when the sun goes down and the sky gets dark, there is no more sunshine to warm the sand. The hot sand starts to cool down, and since there aren9t many plants or water to keep the heat, the desert gets cold quickly at night.
So, the sun warms the sand during the day, making the desert hot, and when the sun sets, the sand cools off, making the desert cold.