Things I wish I didn't hear

I cannot get this out of my mind and I couldn't bear to look. Overheard late this afternoon:

Oldest son, deeply disgusted: Stop biting your toenails!

Second-oldest son, nonplussed: I'm not.

Oldest son, vehemently: Yes you are! I can see you chewing!

Second-oldest son, answering matter of factly and calmly: No. I'm not biting my toenails. I'm chewing off dead skin.

Ewwwwww. Second-oldest son refuses to see this as icky in any way - instead erupts into giggles at our dismay.