How do you tell your parents that there drinking water is unsafe?

This depends - WHY is it unsafe? old pipes? a well that's contaminated? If it's pipes then you might apply a filter or get brita pitchers with charcoal filters. The filters need to be replaced in a couple of months though. If it's a well that's contaminated - that's trickier. You could always call the county health department - OR put a chlorinator on the pipes that come into the house. We used to just dump a gallon of bleach down the well casing once a month - it worked for a while then dad installed a system for all the water coming into the house.