imagine you're at a backyard barbecue and let's say you bring the burgers, and so someone else will be bringing hot dogs, a third person a beer-can chicken to roast, and so forth, and you would like to exchange burgers for an assortment of other options. a single commodity (e.g. burgers) is exchanged with other articles in the most varied proportions. nevertheless its exchange-value remains unchanged regardless of whether it is expressed in x hot dogs, y chicken, z gold, etc. it must therefore be distinguishable from these, its various manners of expression.