Metals can be defined by their properties: they are malleable, so can be shaped by hammering and bending; tough and fairly elastic, so can sustain considerable stress without breaking; and dense and highly reflective, so capable of taking a good polish. They are also good conductors of heat and electricity. With the exception of iron, most common metals have relatively low melting-points (below 1100°C), so they can be easily melted, and on cooling they retain their molten shape. They can, therefore, be shaped by casting. They are also highly reactive, so although abundant in the Earth’s crust, they are usually found in combination with other elements.