How do I explain to a child what self harm is without traumatizing them?

As of recently, one of my younger cousins has started to reach the stage of her somewhat toddler life of questioning everything and always asking random things, like "why is the sky blue? " and whatever.

she is about 6 years old, a happy go lucky girl and very VERY curious, and since I've started to be able to interact with her more since she now somewhat grasps what I can say and we can talk about her favorite shows and songs and her favorite games on her tablet and stuff, BUT she has been able to see my scars more often, theyre not THAT visible on my forearms but on my upper arms they're VERY visible, I am very pale and the scars are deep and red and purple and there's may of them, and since its summer/early autumn now, I still wear short sleeves.

She didn't question it before, she always just stared kind of, until recently. So for the past few months she's been asking about them almost every time my aunt and uncle come over about what those marks are, and I always try to brush her off and say " I'll tell you some other day! " or " I'll tell you next time" or "I'll tell you when you're older! " and anytime I do say that she asks why I can't say it now and keeps on questioning everything, typical little kid stuff I figure. She asks why I have those and her mom doesn't, or what are those things? and some times she's tried to wipe them off with a paper towel for some reason???

But anyway I don't know how to get her to stop asking, I have thought about the typical " I fought with a tiger! " option but I don't want it to be a possibility for her to brag about it to her friends and possibly worry some other adults. . . but it might just be me over thinking.

so how do I go on about this? do I create a story about me fighting some beast? or do I say " oh these started appearing when I was younger and got really really sad! " or something?? what do I even do?? I don't want to traumatize a little kid on accident or say something that her parents might hate me for, self harm isn't a thing my family ever acknowledges