My father plays psychology games with my mother to stress her out.

I've always noticed this since my childhood. And yesterday something happened which I would like to share. My mother went to the terrace with some keys and suddenly the keys fell down. She asked my father to pick them up. He went down and she was overlooking him from the terrace. She saw him pick up something and put it in his pocket. Then she went down and met him in the stairs and asked if he found it. He made a very serious face and said "No!". She obviously asked "Then what did you pick up?". At this, he just laughed and said he wanted to teach her a lesson ( as if she is a child ). This may seem a normal fun act from my father's side. But it's not. What he intended was to stress her out like he has always done. My mother already has so many mental health problems like depression, anxiety, etc.