Collectively known as the thermal-fluid sciences, this cornerstone of mechanical engineering integrates three closely related disciplines to analyze and design systems involving energy and fluids. Thermodynamics deals with energy, its conversion between different forms like heat and work, and the properties of substances, setting the theoretical limits for system performance. Fluid mechanics is the study of how fluids (liquids and gases) behave under various forces, providing the principles to understand flow, pressure, and lift. Heat transfer complements these by focusing on the rates at which thermal energy is exchanged between physical systems through conduction, convection, and radiation. Together, these fields are essential for designing everything from engines and power plants to HVAC systems and aerospace vehicles.