if something is a source of something else then that something usually contains that something else & food is a source of energy for living things ->  food contains energy for living things  food contains energy for living things & food contains nutrients for living things ->  food contains nutrients and energy for living things  food contains nutrients and energy for living things & to contain something can mean to store something ->  food stores nutrients and energy for living things  food stores nutrients and energy for living things & the ability to store something is a kind of property ->  the ability to store nutrients and energy is a property of food  the ability to store nutrients and energy is a property of food & if something does not have the properties of something else then that something is not that something else -> if something does not have the ability to store nutrients and energy then that something is not food