After all the dinosaurs died, where did the mammals come from?
There were there all the time. Mammals had already split off from their common ancestor with reptiles long before the dinosaurs appeared. From the beginning of the Triassic, through the Jurassic and the Cretaceous, mammal ancestors were busily trying to survive. Since the dinosaurs leapt into all the huge animal jobs, the pre-mammals were left all the little scurry-around positions that no one wanted. When the Cretaceous extinction surprised everyone, the little mammals were somehow able to survive, possibly by a combination of hiding, needing less food, having high reproductive rates (lots of kids), and being able to take advantage of any adaptions rapidly (by having lots of young).
