Mushrooms grow with astonishing speed, and the fully-grown mushroom often looks quite different from the baby version. The photos below are both amanitas. As the mushroom grows, the universal veil usually leaves patches stuck to the cap, and the partial veil (which encloses just the gills), often leaves a ballerina-like skirt around the stalk. And just so you know, these are poisonous.
This is a different species, but it grew nearly as fast (I was away for a day, so it might have been less time than my photos show). But the transformation is even more dramatic, from something that looks like a cottage-loaf (or a puffball), into a true toadstool.