Mark and Spencer have very different lifestyles: Mark makes more money than Spencer, likes pop music, and is a vegetarian. Spencer is more of a heavy metal fan and his favorite food is steak.
However, they have one similarity: both of them buy their groceries in the same store. Both of them consume some baked beans per week, although Mark eats more due to his "condition" (vegetarianism). And both of them drink a few sports drinks per week, although Spencer consumes more to recover from all the headbanging.
Apart from the store where they shop, and therefore prices that they face, what must be the same for both of them?