I can use functions when I am at the fair. When I go to the fair I see a sign that says, 1 ticket $5. 2 tickets $7. 3 tickets $15. 4 tickets $25. So I  go to the fair, and I see that sign. I have four people with me. I could see that it would be better to buy two sets of two tickets, each for $7, than to buy four tickets for $25. Simply because it would cost less.