One day the rote example finally made sense to me, and I go back to it every time I hear about variance.

Got a Container<Animal> and want to treat it as a Container<Cat>? Then you can only write to it: it's ok to put a Cat in a Container<Cat> that's really a Container<Animal>.

Reading from it is wrong. If you treat a Container<Animal> like a Container<Cat> and read from it, you might get a Dog instead.

The same works in reverse, treating a Container<Cat> like a Container<Animal> is ok for read, but not for write.