What is gravity and why do things fall down instead of up?

Drop a sock, a spoon, a slice of toast — they all rush toward the floor like the floor is having a party and everything's invited. Nothing ever wanders sideways and floats off into space. There's a quiet rule running the whole show, and its name is gravity. So what is it, and why does "down" win every single time?
Here's the first surprise: gravity isn't a force that lives in the ground sucking things down. It's a pull that every object with mass has — the more stuff something is made of, the harder it tugs on everything around it. You have gravity. A coffee mug has gravity. A mountain has gravity. It's just that some pulls are too tiny to ever notice.
