The first principle you need to understand about geologic time is that the laws of nature are always the same. This means that the laws describing how things work are the same today as they were billions of years ago. For example, water freezes at 0°C. This law has always been true and always will be true. Knowing the natural laws helps you think about Earth’s past, because it gives you clues about how things happened very long ago. It means that we can use present-day processes to interpret the past. Imagine you find fossils of sea animals in a rock. The laws of nature say that sea animals must live in the sea. That law has never changed, so the rock must have formed near the sea. The rock may be millions of years old, but the fossils in it are a clue for us today about how it formed.