Thermal expansion, heat transfer and energy sources

Heat is transferred in three ways. In conduction heat passes through the body itself, particle to particle, from the hot part to the cooler part (a spoon warms in hot tea). Metals (copper, aluminium, iron) conduct heat well; wood, plastic, glass, brick, air and straw conduct it poorly and are thermal insulators. In convection heat is carried by the currents of a liquid or gas: warmed air or water becomes lighter and rises while cooler heavy layers sink (that is why a radiator is placed under a window, near the floor). In radiation heat travels as rays, even through empty space: this is how the Sun warms the Earth, and we feel it near a fire. A thick coat does not warm you by itself but prevents body heat from escaping (its pores hold air). In a greenhouse glass lets sunlight in and stops the warm air from escaping. Brick and adobe walls and a straw-and-clay roof keep a house cool in summer and warm in winter. Safety: when holding hot pots or spoons do not touch the metal part that conducts heat well.