It is how they know, when in the deepest, most solid sleep, that you're having sex or masturbating, even if it's at the other end of the house. And so choose that time to wake and start crying. We haven't experienced this yet, but have it on good authority.
It is how they can tell you're in your nice, expensive, going-out clothes, and will choose that moment to vomit all over them. We haven't experienced that one either, though we have had Lex vomit all over himself just as we got him out to the car.
It is how they know when you're just sitting down to eat, or to watch something, and decide to wake and need instant attention.
Lex has the uncanny ability to completely sleep through and ignore Sharon doing all sorts of noisy stuff. But should Sharon try to lay down for a sleep or rest, he will wake and want attention. He does the same thing when she eats.
They do not grow out of this, their targeting system shifts slightly from 'annoyance' to 'embarrassment.'
It is how you explain their capacity, in later years, to unerringly find your vibrator and bring it out to show when your maiden aunt is visiting. "Look, a rocket!" they will proudly proclaim while making whooshing noises.