Gravity

So it's New Years eve and my wonderful wife and I are driving our usual 4 hour drive to Show Low so that we don't have to stay in Tucson in our sardine can and so that we can go cross country skiing, which Roberto otherwise known as Wart introduced us to. On this drive you have to go through this treacherous canyon called the Salt River Canyon. It is bad because of the many rocks that fall down the side of the mountain(because of gravity) and end up in the road where unsuspecting motorists hit them often and blow out their tires. So here we are coming out of the canyon and after I had just passed a car and was going around a corner I had to lower my headlights as a truck was coming the other way. Low and behold right there was a rock that looked like it was the size of a basketball which I promptly ran over with both left side wheels. Sharon screamed as the impact was so loud and she thought the truck had hit us. I am thinking at this point that we will probably be sleeping by the side of the road as my tires are probably both toast. But low and behold we are still going. I pulled over into the first turnout and got out and they were both still round with no sound of escaping air. The front rim was pretty flat in one area though and at low speeds the tire bounced pretty good. So my Irish luck was with me and we proceeded to Show Low in a timely manner especially after I got up to 65 or 70 mph and the tire smoothed out. So gravity got me. So what is gravity anyway? If you ever wipe out an aluminum rim there are places that will fix them for $100 which is pretty good considering that they are $400 new. So I really want to know, what is gravity? The dictionary doesn't explain it well. Also I wonder, where does the ocean water go at low tide? And why do we have bald eagles that come here to Arizona for the winter? They must be true "Snowbirds." I think they are all vagaries of nature.