Metals and non-metals are defined by their properties. Metals are good conductors of heat and electricity, unlike non-metals. Metals are all solids (with the exception of Mercury). Metals have a high melting point. Non-metals have a low melting point. What this means is that non-metals will turn to a liquid at a lower level of heat than metals.
For example, iron does not turn into a liquid until it reaches a temperature of 2,800 degrees Fahrenheit (1,538 degrees Celsius). Sulfur, a non-metal, has a melting point of 239 degrees Fahrenheit (115 degrees Celsius).