Apparently, supermarkets offer discount for multiple purchases, so sometimes you can save money by buying shampoo and conditioner at the same time. Then, when either the shampoo or conditioner run out, throw the other bottle away because you always have to use matched shampoo and conditioner and don't even think about buying another bottle of whichever has run out because it won't be on offer and anyway you have to regularly change the shampoo you use because otherwise it builds up deposits on your hair. WTF?