steam is hot in temperature & a hot substance is a source of heat ->  steam is a source of heat  cooking food requires adding heat energy to the food & a source of heat can be used to heat something ->  a source of heat can be used to cook food by adding heat energy to the food  steam is a source of heat & a source of heat can be used to cook food by adding heat energy to the food ->  steam can be used to cook food by adding heat to the food  steam can be used to cook food by adding heat to the food & adding heat to something means trasferring heat to that something -> steam can be used to cook food by transferring heat to the food