Thermodynamics is the study of energy, but to study energy, we first need to define what we're looking at. Imagine a cup of hot coffee on your desk. This cup of coffee can be our 'system'—the specific part of the universe we want to focus on. Everything else, from the air in the room to the desk it's sitting on, is the 'surroundings.'

The boundary between the system and its surroundings is crucial. It determines how the system interacts with everything else. We can classify systems into three types based on what can cross their boundaries.

• An open system can exchange both energy (heat and work) and matter with its surroundings. Your cup of coffee is an open system. It radiates heat into the room, and water vapor (matter) escapes from the surface.
• A closed system can exchange energy but not matter. If you put a lid on your coffee, it becomes a closed system. It still cools down by releasing heat, but the water vapor is trapped inside.
• An isolated system cannot exchange energy or matter. A perfectly insulated, sealed thermos would be a good approximation. In reality, truly isolated systems are hard to create, but they are a useful theoretical concept.