A thermometer works by exploiting the principle of thermal equilibrium. It typically consists of a glass tube containing a liquid, often mercury or alcohol. When the thermometer is placed in contact with a substance, heat is transferred between the two. If the substance is hotter than the liquid in the thermometer, heat flows from the substance to the liquid until they reach the same temperature. Conversely, if the substance is colder, heat flows from the liquid to the substance until they reach thermal equilibrium. The key is that the rate of heat transfer into the liquid equals the rate of heat transfer out of the liquid. The volume of the liquid expands or contracts with changes in temperature. This expansion or contraction is mechanically linked to a scale, allowing us to read the temperature. The thermometer achieves thermal equilibrium by allowing heat to flow freely between itself and the substance until their temperatures are equal.

The metal block and the water will eventually reach a constant temperature because they will establish thermal equilibrium. This occurs when the rate of energy transfer from the hotter object (the metal block) to the colder object (the water) equals the rate of energy transfer from the colder object to the hotter object. Initially, the metal block is hotter than the water, so heat energy is transferred from the metal to the water. This transfer continues until the metal and water reach the same temperature. At this point, the rate of heat transfer from the metal to the water becomes equal to the rate of heat transfer from the water to the metal. This is because the temperature difference between them is now zero, so there is no further net transfer of energy. The rate of energy transfer is dependent on the temperature difference between the objects and the thermal conductivity of the materials involved. A larger temperature difference and higher thermal conductivity will result in a faster rate of heat transfer. The system continues to exchange energy until the temperature difference is eliminated, resulting in a constant temperature for both the metal block and the water.

The Earth's temperature is largely determined by the balance between the energy it receives from the Sun and the energy it radiates back into space. Incoming solar radiation primarily consists of short-wave radiation (visible light and UV). The Earth's surface absorbs some of this radiation, warming the surface. The warmed surface then emits energy as long-wave radiation (infrared radiation). Greenhouse gases in the atmosphere absorb some of this outgoing infrared radiation, preventing it from escaping directly into space. This absorption process re-radiates infrared radiation in all directions, including back towards the Earth's surface. This trapping of infrared radiation is known as the greenhouse effect.

Increased concentrations of greenhouse gases enhance the greenhouse effect. Here's how specific gases contribute:

• Carbon Dioxide (CO2): CO2 is a major greenhouse gas produced by the burning of fossil fuels, deforestation, and volcanic activity. Increased CO2 concentrations lead to greater absorption of infrared radiation, resulting in a warmer Earth.
• Methane (CH4): Methane is a more potent greenhouse gas than CO2 over a shorter time scale. It is produced by livestock, agriculture (rice paddies), and the decomposition of organic matter. Increased methane concentrations significantly enhance the greenhouse effect.
• Nitrous Oxide (N2O): Nitrous oxide is produced by agricultural activities (fertilizer use), industrial processes, and the burning of fossil fuels. It is a very powerful greenhouse gas with a long atmospheric lifetime, contributing significantly to global warming.

Therefore, an increase in the concentration of greenhouse gases leads to more infrared radiation being absorbed and re-radiated back to the Earth's surface, resulting in a higher average global temperature. This disruption of the energy balance is the primary driver of global warming.