My mom had a similar rule: “You cannot play hairdresser with real scissors and your sister’s hair”. “You can’t play haridresser with real scissors and your hair” followed soon enough.
I am the Meanest Mommy in the Whole Wide World. I am forced to make rules & observations that I never knew would be necessary before I became a parent.
Rules by the Meanest Mommy in the Whole Wide World 
My mom had a similar rule: “You cannot play hairdresser with real scissors and your sister’s hair”. “You can’t play haridresser with real scissors and your hair” followed soon enough.
My son did the same thing with my husband’s beard trimmer!! Twice!! Hubby now takes the blade off when he’s done.