... measures the effects of the destruction of germs in PET bottles through exposure to UV radiation. In the 'Sodis' (Solar Water Disinfection) method, water is placed in a transparent PET bottle and left in the sun for six hours. The UV light contained in the solar spectrum kills germs and pathogens. However, clouds or a scratched bottle prolong the required cleaning time, so that people consume germ-contaminated water after six hours and fall ill. The students therefore developed a reusable, wear part-free indicator that monitors the cleaning process in the bottle and interacts with the user in a simple way via pictograms. This prevents misuse and the resulting illnesses and mistrust of the cleaning method. It also makes the method more accessible for children and illiterate people.