How your body works? A knee jerk reaction!

I react to unusual yelps of pain. I find her in the family room, beating herself with a hair brush, with a silent audience, her brothers.
“What on earth are you doing to yourself?”
“I can’t make it work!”
“Make what work?”
“The knee jerk thing.”
“Oh……I see. Well you don’t have to be so violent about it. It’s really easy. Let me show you. Pull up your trouser leg. Cross your legs. Now feel your knee cap and tap firmly just underneath.”
“Wow! It works!”
“Be more gentle. It only takes a tap.”
“Geez, that is so cool!”
“You can do it to anyone, especially if they have bare legs. All you have to do is look carefully and tap. It makes for a great joke. You could try it at school.”
“Come on you guys. Whydontcha have a go too?” The boys examine their own knee caps, with care and a good degree of contemplation.
“Not joke.”
“It’s not a joke? Don’t you think it’s funny dear?”
“Er…….not if you ain’t got no legs.”
Rats to the "theory of mind!"