Metals are good conductors of heat and plastic is a bad conductor of heat. Convection is when heated fluids expand and so become less dense. Convection is transfer of heat energy, within a fluid, by the circulation of the fluid due to temperature differences. The currents of moving molecules are called convection currents. Convection occurs in fluids because the molecules in solids are not free to move.
Heat travels through the air by convection.
If the heat is allowed to do these things the liquid is bound to cool down because the heat will be able to travel through the air by convection and the heated molecules will escape. I think that if I stop these things from happening I will be able to keep the liquid hot. I think I could do this by insulating the beaker in which the liquid is stored. If I do this convection will not be able to happen. Also I think that if hot liquids are put in different size cups but they have the same amount of water in, the cup which is the largest will lose the most heat, because the hot liquid will be spread over a very large area and when the heat travels by convection more molecules will escape, because they have more space and less depth of liquid to do so.