Thermal convection refers to the phenomenon that heat is transferred by the flow of liquid or gas. Among the following phenomena, the thermal convection phenomenon is. ['In the winter, you can feel the iron rods and wooden sticks of the same size with your hands.', 'Snow covers the ground in winter, but winter wheat is not easy to freeze to death.', 'Hot summer, people like to wear light clothes.', 'Place the paper windmill on the top edge of the lampshade, and the windmill will rotate.']