A theory is a well-substantiated explanation of some aspect of the natural world, based on a body of evidence. It helps us understand complex phenomena by providing a framework for interpreting observations. For example, the theory of evolution explains how species change over time through natural selection, offering insights into the diversity of life on Earth. Theories are not just guesses; they are built on rigorous testing and validation. Scientists use theories to make predictions and guide research. For instance, the theory of relativity by Albert Einstein transformed our understanding of space and time, showing how they are interconnected in ways we experience every day.