Think about it. That little boy who's running back into his house, he may have a nice, clean fresh water supply, but he's got dirty hands that he's going to contaminate his water supply with.
Pensateci. Quel bambino che sta correndo verso casa, potrebbe avere una bella fornitura di acqua fresca e pulita ma ha le mani sporche e contaminerà la sua fornitura di acqua.