The girls are studying astronomy this year, and we've just finished up a chapter on Mercury. As a hands-on project, I made a ball of salt dough. The girls had a ball pretending that marbles and pencils were asteroids slamming into the planet. Even Lauryn got into the action.
After drying overnight, the planet has hardened. As Mercury has no atmosphere, it has no protection against space debris. However, there are parts of Mercury that are relatively smooth. This has most scientists confused, because after "millions of years", it "should" be completely pock marked.
If you are ever looking for a creation based, young earth worldview science elementary curriculum, please take a look at the Elementary Apologia books. It's science is solid, but geared age appropriately, all the while giving God the glory for the creation we are studying.