It's a tiny, liquid popping sound.
I hear it every time I look at these pictures of the first time my boy met my girl.
I will never, ever forget how, the first time he ever saw and held her, when he would have had every right to be standoff-ish, threatened, squirrely or even outright mad ...
Instead, he smiled. And kissed her head. And handled her as if she were made of glass and he wanted to protect her.
I am blessed beyond measure in my children.