How do you date a fossil? You could ask what their favorite mix tape was? Oh, wait, wrong app. It’s #NationalFossilDay! (Do not come for us for the mix tape comment.) Did you know today marks the 15th anniversary of National Fossil Day? Rock on! Back to dating fossils. There are two different methods to age-date fossils. Relative dating simply tells us which of two fossils is older, and which is younger. Fossils are most commonly found in sedimentary rock, which forms in layers. The layers at the bottom formed first and, therefore, are older than those above them. For example, dinosaurs are found in layers that are below those containing the first humans, so we can conclude that dinosaurs and humans did not live at the same time; the big dinosaurs like Tyrannosaurus and Triceratops lived and died long before humans first evolved. Absolute dating allows scientists to determine exactly how long ago an organism lived. Certain elements in rocks are radioactive. These elements are unst...