This is by far the most common argument I hear in my daily life from those within my social circle. It is, in essence, an attempt to establish reasonable doubt rather than refute the science. It fails miserably.
Here is my reply.
You check your general savings account and notice a large drop in the balance from what you know should be in there.
You go into a branch of your bank to question them about it. The teller looks at your account and confidently concludes, “Your account balance always changes.”
Is that the end of the story? Would you accept, on any level, that this reply answers your concern?