Dinosaur and Human footprints?

I am an atheist, and I don't know how to explain this. Can an athiest who knows fill me in on how this is possible. The only thing I can think of is the layers have been shifted over time so they overlap or something. Maybe similar to N transforms in archaeology. Anyway does anyone have the scientific reason behind these footprints. I would like to know incase a christian brings it up in an argument against me haha!