bad for the screen

“Look”, she says.
My mother presses her index finger
Against the flat screen monitor.
She looks back at me smiling
Like a child.
She shows me how the image on the screen
Distorts around her fingertip
Like ripples in a pond.
“Mom! Don’t do that!”, I tell her. “It’s bad for the screen.”
I think I heard that somewhere...
“Oh!”, she says, startled, the rare innocent wonder wiped clean from her face.
But why did I have to say anything?
Why couldn’t I just let her have that?