Does your dog really know what sit mean, or is it heavily context oriented?
Here are some fun tests you can do with your dog.
1. Ask your dog to sit when he is in a "down" position.
2. Sit cross legged on the floor with your hands between your legs and ask for a sit.
3. Turn your back to your dog and ask for a sit.
4. Ask your dog to sit when it's on the bed.
5. Walk away from the dog and ask for a sit.