It's wrong to break a pay it forward chain.
Katy is at the supermarket and a stranger offered to pay for her groceries saying a stranger had done something nice for him earlier.
Katy wants to get her free groceries home.
Katy thanks the stranger for the food and pays for the next strangers's food who was behind her in line before heading home.
Katy drives home and tells her brother about the stranger, and they both agree it is quite odd to do that sort of thing.
Katy feels good for passing along the gesture towards somebody else.
Katy never gets to feel what it is like to help a stranger out and stops a pattern that had been going on.