Sit down! When you sit, you rest your weight on your bottom instead of standing on your feet. You can sit in a chair, sit on the floor, or sit in a giant bowl of chocolate pudding if you want to.
When you tell a good dog to sit, she lowers her hindquarters to the floor, with her back legs bent. But when you sit ten people at your little kitchen table, you provide seats for them, and when you sit for an artist, you pose while she paints or draws your portrait. Objects sit by simply not moving: "The cars sit, stuck in rush hour traffic, while the subway speeds past."