How Fossils Form.
Fossils have come up already on this site — shale and limestone both mention preserving them — but never got their own explanation. Worth a short standalone page, since it's a natural, high-interest companion to the sedimentary rock content.
A fossil is the preserved evidence of ancient life — not usually the original organism itself, but a record of it left behind in rock. Most fossils form through a process that depends entirely on sediment: an organism (or a footprint, burrow, or other trace it leaves behind) needs to be buried relatively quickly, before it can fully decompose or be scavenged. That's why fossils are overwhelmingly found in sedimentary rock rather than igneous or metamorphic rock — the same slow accumulation of mud, sand, and sediment that builds sandstone, shale, and limestone is exactly the process that can also bury and preserve something living.
Once buried, a few different things can happen. In permineralization, the most common way hard structures like bone and shell fossilize, mineral-rich groundwater slowly seeps through the buried remains, and minerals crystallize within the tiny pore spaces of the original material, essentially turning it to stone from the inside while preserving its original shape and structure in extraordinary detail — this is how most dinosaur bones and petrified wood form. In other cases, the original material dissolves away entirely after burial, leaving an empty mold in the surrounding rock in its exact shape; if that mold later fills in with new minerals, it becomes a cast — a stone copy of the original, rather than the original material itself. Trace fossils aren't the organism at all, but evidence of its activity — footprints, burrows, even fossilized droppings — telling a story about behavior rather than anatomy.
Why calm, quiet environments preserve fossils best: fast burial and low oxygen both slow decomposition and reduce scavenging, which is exactly why shale (formed in still, calm water) and limestone (formed in shallow marine environments rich in shelled organisms) are two of the best fossil-bearing rocks on this site's list — it's the same "how it forms" story, just told from the perspective of what got trapped inside while it happened.
Worth linking: back to Shale and Limestone's own pages (where fossils are already mentioned), and to the Rock Cycle, since fossils are ultimately just another kind of evidence riding along inside the sedimentary half of that cycle.