STOCK :-Those materials in the environment which have the potential to satisfy our needs but we do not have the proper technology to access these materials, come under generated in sufficient amount of stock.For eg. Water is a compound of two imflammable gases ;hydrogen and oxygen. These can be used as a rich source of energy. but we do not have the required technology to use them for this purpose. So, water can be considered as stock RESERVE:-Which can be put into use with the help of existing technology but their use has not been started . These can be used for meeting future requirements .River water can be used for generating hydroelectricity power but presently , it is being used only to a limited extent.So, we can say that water in the dams , forests , etc. is a reserve which can be used in the future.