An example of a fossil is a footprint in a rock. Lisa was hiking in a rocky area and in a rock, she saw what looked like footprints made by a bird. But weirdly, they were in a rock. Where did they come from? ['The footprints were originally in mud or peat, but after a lot of time, the substance had been changed into rock', 'The rock was actually an asteroid', 'a very heavy bird made imprints in the rock.', 'a spaceship used a laser to make the etches in the rock']