When you heat a Bowl of water, We can observe that water starts circulating. This demonstrates :

When you heat a bowl of water, we can observe that the water starts circulating. This is nothing but convection. As when water is heated, the water molecules above the surface of bowl is heated first; temperature increases and density decreases; and due to decrease in density; that water molecules goes up and water molecules present at brim goes down because of relatively more density. Due to this water circulates, this is convection.