Please answer the following question: I want to test the ability of students to read a passage and answer questions about it. Could you please come up with a good question for the passage "Solar water disinfection (SODIS) involves exposing water-filled plastic polyethylene terephthalate (PET) bottles to sunlight for several hours. Exposure times vary depending on weather and climate from a minimum of six hours to two days during fully overcast conditions. It is recommended by the World Health Organization as a viable method for household water treatment and safe storage. Over two million people in developing countries use this method for their daily drinking water."?
A:
What do many people do to their water before storing it?