Imagine you live near a lake. At some point you detect lead in the lake and you notice your friend upstream is putting out a lead-containing compound in the lake.
You tell him to stop because he is making the lake toxic. He says: But lead is a natural compound, you should expect to find it in the lake.
So you show him the amount of lead has been increasing for the past months closely correlated with the amount of lead he has put into the lake.
So he says: But that's not proof. It might be some other source of lead which is poisoning the lake. So you show him computations which show the amount of lead he is putting into the lake relative to the body of water can be expected to increase the amount of lead in the lake.
So he comes back with a glass of water and tells you: Look, no lead in this water. So you ask him where he got the water from and he says he got it right near a small stream leading into the lake after a rainfall. So you tell him your claim is not *every part* of the lake has gotten more lead in it, only that on average the amount of lead has gone up.
So he tells you how you can know that lead is bad for life in the lake? You show him medical evidence for this. He tells you that if the lead is bad for the fish, it's also bad for the mosquitoes, and that's good insofar he is concerned.
Then you give up trying to convince your neighbor and accept the lake will be poisonous.
