If you return other people’s bottles because they are willing to forego the deposit, then you do make some money. One of the ways that I survived my time as a skint foreign student in a Nordic country was picking up bottles in the city center on a Friday or Saturday night when everyone was drinking in the street and couldn’t be arsed to hold on to their bottles and take them back for the deposit. (I hear that now students find it harder to make money like this, because Roma immigrants have cornered this line of work.) Even later when I wasn’t so poor, there is pleasure in hosting a BYOB party and then being able to get the deposit for all the bottles and cans your guests brought.

Here’s one from my student days:

We define the derivative of a party as the amount of alcohol that can be purchased by returning the empty bottles left after the party. A good party is a party such that the second derivative of the party is positive.