Here's why:

* Solar Radiation: The Sun emits electromagnetic radiation, including visible light, infrared radiation (heat), and ultraviolet radiation. This radiation travels through space and reaches Earth.

* Absorption and Emission: The Earth's surface, including land and water, absorbs some of this solar radiation. This absorption causes the surface to heat up. The heated surface then emits its own infrared radiation back into the atmosphere.

Of course, there are other sources of heat on Earth, like geothermal energy, but they are significantly less impactful than the Sun's radiation in terms of heating the land and water.