Convection only happens with fluids, materials that can flow. Liquids can flow (think of water) and gases can flow (think of wind). Solids are stuck in place so they can't flow and since they can't flow, there is no convection. The way heat transfers through solids is conduction.

Transfer of Heat by Convection in Fluids
Convection is a mode of heat transfer that occurs only in fluids, such as liquids and gases, and not in solids. This is due to the nature of fluid molecules and their ability to move freely.

Fluid Movement
In fluids, the molecules are not fixed in a specific position like in solids. They are able to move around and flow from one place to another. This movement of fluid molecules allows for the transfer of heat by convection.

Hot and Cold Fluid Movement
When a fluid is heated, its molecules gain energy and move faster, becoming less dense. This causes the hot fluid to rise, while the colder, denser fluid sinks. This creates a circular motion within the fluid, known as convection currents, which help in transferring heat.

Transfer of Energy
As the hot fluid rises and the cold fluid sinks, they carry heat with them. This continuous movement and exchange of hot and cold fluid help in transferring heat from one place to another in the fluid.

Conclusion
In conclusion, the transfer of heat by convection takes place only in fluids because of the ability of fluid molecules to move freely and create convection currents. This mode of heat transfer is crucial in processes such as cooking, heating, and cooling in everyday life.