I go to the grocery store multiple times a week and see different prices for things I buy. They are constantly changing prices, with sales and offers and 2-for

I go to the grocery store multiple times a week and see different prices for things I buy. They are constantly changing prices, with sales and offers and 2-for-1. I know this is to confuse me into not knowing what the "real" price of a good is, so that when I run out of ketchup I'm somehow fine paying $8 for a bottle.