The mall rats have decided that you're an ok guy/gal/both? and have permitted Rod to trade more expansively with you. He will now accept offerings of water, and let you sample the delicious stockpile of junk food he has. Yum!
PS: The mall rats will accept dirty water too. Infected water has no effect on their already stabilized and ratty forms. Some even say it has a nice tangy taste to it that they like.
