Science is a way to get knowledge. Like all knowledge gathering, science starts with experience and takes facts and rules from it.
Science involves studying things by looking at them very carefully, by measuring them, by doing experiments and tests, and by trying to explain why things act the way they do. People who do these things are called scientists.
Scientists must be very careful to create explanations that fit well with what they observe and measure. They compete to provide better explanations. An explanation might be interesting or pleasing, but if it doesn't agree with what other scientists really see and measure, they will try to find a better explanation.
Scientists also try to check the ideas of other scientists. Since scientists are people, sometimes they make mistakes or think that a certain explanation sounds good, even if it hasn't been checked well enough. Other scientists check their ideas to see if the real observations, measurements, and tests show if they are right or wrong. Peer review and repeating experiments is the only way to be sure the knowledge is correct. This is the scientific method. Most people accept it as the most reliable way to get knowledge about nature, the universe, human bodies and those of animals.
Science produces models of nature, models of our universe, and medicine. There are many different sciences with their own names. However it is not right to say "science says" any one thing. Science is a process, not just the facts and rules believed at one time.