Katie conquers stairs

Where does an apartment dwelling baby learn to do stairs? I actually always wondered this - not only where, but how do they think to do it when stairs are not part of their daily experience. Yesterday I learned that it's just from exploring. We were waiting in the opthamologist's office and Katie crawled around (it's a pediatric practice, very big and kid friendly) and she just made her way to the 2 steps and crawled right up. She did this over and over again, but not down yet, that will take a little more practice.