When you sit down to read some novel, please do not think that you can simultaneously carefully supervise your child with the big-girl scissors. Especially do not assume this if she has long beautiful brown hair. You might look up to check on her, and find that she has already snipped three or four locks of her hair off, and is working on the next one. And then you might have to trim bangs for her, and she might think she is the cutest thing since the Kewpie Doll.
And you will have to agree with reservation, because you don't want her to think it's okay to just trim her own hair whenever she wants.
Also, if your sister comes over, don't let her talk to the deviant child, because she will be unable to refrain from smiling, laughing, and talking about how cute the child is with her new bangs, utterly removing the residual stern reprimands you have carefully applied throughout the day.