cause

There is an old joke where a young man takes a job selling mouthwash door to door, and is given a suitcase full of 100 bottles priced at five dollars per bottle on his first day. He is one of three new salespeople hired that day, and all of them are nervous to do well. At the end of the day, the manager has the salesmen lined up, and asks them all to let him know how many bottles they sold today. The first salesman admits to only selling five bottles, and the manager simply lets him know it will take a little time before he gets good at it. The second salesman admits to doing slightly better at fifteen bottles, and the manager is pleased. The manager asks the young man how he many he sold, and he opens his suitcase which is now empty and tells the manager he has sold all of them. Surprised, the manager asked him how he could have possibly sold all of the mouthwash in one day. The young man said he simply set up a table by the side of the road and set out doughnuts filled with coal, and next to that he set out the bottles of mouthwash. Customers would then come buy the doughnuts, which were priced at twenty five cents, and when the customers complained that it tastes like coal, he would simply ask them if they would like to purchase a bottle of mouthwash. It goes without saying that all of the customers purchased a bottle of mouthwash with their doughnut.