"Nevada!" I answered, with equal enthusiasm. "It looks like Nevada!" And it did.
We'd been playing Scrambled States of America (affiliate link), one of my kids' favorite games, a few nights before, and when my daughter spotted this particular rock, she remembered the shape of the state it resembled and the discovery excited us both.
Of course I have an atlas!
Remembering that I had an atlas in the van, I went and got it and gave it to her, asking if she'd like to look for more rocks that look like states.
"Yes!" she exclaimed and ran off, rock and atlas in hand to search for more.
By the end of the afternoon, she'd found at least half a dozen states, and wanted to keep looking, but it was getting too chilly and the sun was going down, so I promised to take her and her siblings down to the creek the next day to continue the search.
A whole week of geography

My daughter was so happy with her finds, she asked if she could paint the rocks to match the states on the map.
"Sure!" I said, really amazed that she'd managed to essentially come up with a whole unit study on geography all on her own.
We'd be working on learning about states for at least a week, maybe more, and she'd have some pretty cool souvenirs to keep for a long time to come.
"This is homeschooling," I thought. What had been a day off from planned studies turned into an amazing learning experience encompassing geography, reading and art that I would have never thought to introduce, but those unplanned homeschooling adventures are the very best part of this form of education.