A number sentence is like a story problem, but instead of using words, we use numbers and signs like plus, minus, multiply and divide. It is a way of writing a calculation or math problem in a clear and organized way. Imagine you have 5 cookies, and your friend gives you 3 more, you can write it as "5 + 3 = 8", which makes a number sentence. This helps us understand and solve math problems easily. So, just like we tell stories using words, we tell stories using numbers with a number sentence to solve math problems.