* Fluid properties: This means the substance can flow and change shape. This includes liquids, gases, and some semi-solids like molten rock.

* Temperature differences: Convection requires a difference in temperature within the substance. Warm parts of the fluid rise, while cooler parts sink, creating a cycle of movement.

Here are some examples of substances where convection occurs:

* Air: Convection in air is responsible for weather patterns, like thunderstorms and sea breezes.

* Water: Convection in water drives ocean currents, which distribute heat around the globe.

* Molten rock: Convection within the Earth's mantle drives plate tectonics and volcanic activity.

* Plasma: Convection is a key process in stars and other plasmas.

* Boiling liquids: Convection is why bubbles rise in boiling water.

Things that convection cannot happen in:

* Solids: While heat can be transferred through solids via conduction, they do not have the fluid properties needed for convection.

So, in short, convection occurs in fluids (liquids and gases) where there are temperature differences.