The growth of the baby rat changes over time. To see if it has grown, you have to measure from its head all the way to its tail, and you also measure its weight. By doing that, you can see how much has the baby grown. My science class hasn't really done this because we don't have baby rats. But I have gone to Mr. Kriste's classroom and play with his baby rats. I saw them when they were first born. All the babies were sleeping and their eyes weren't open. But after 5 or 6 weeks, I went back to visit them. They had fur on them and they were playing wildly. I was amazed by how fast the baby rats grew from just being small tiny babies.