How can scientist tell what time period something is from if the world is constantly changing?
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Change is how we measure time. A scientist can calculate the age of a rock by looking at traces of radioactive isotopes, and what they decay into, to determine how old it is. Other times, they can look at things that follow regular repeating patterns like tree rings and layers in ice cores. Building from this, you can tell about how old a sedimentary layer is by what fossils appear in it, since we have have figured out when some organisms appeared and went extinct. Archaeologists can know when a culture used certain kind of tool or practiced a certain art style.
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