[simondw]

But... cows do drink cow milk, that's why it exists.

[lazide]

You’re likely thinking of calves. Cows (though admittedly ambiguous! But usually adult female bovines) do not drink milk.

It’s insidious isn’t it?

[hinkley]

If calves aren’t cows then children aren’t humans.

[miroljub]

So, this joke works only for natives who know that calf is not cow.

[kelnos]

Colloquially, "cow" can mean a calf, bull, or (female adult) cow.

It may not be technically correct, but so what? Stop being unnecessarily pedantic.