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.
Pensez-y. Ce petit garçon qui rentre chez lui en courant, il peut bien avoir à sa disposition une eau propre et pure, mais il a des mains sales qui vont contaminer l'endroit où il trouve son eau.