Boys Will be Boys

Ok, so you know that I live in a house full of boys and as I watch them grow into young men there are a few things I am trying to understand about their gender. First is this: fingernails. Yes, fingernails. DS1 had a bath on Sunday night just before bed, went to school on Monday morning and returned from school with BLACK underneath his fingernails. When I asked him about it he responded, "I don't know how that got there." DH says, "He's a boy." I understand this, I do, and if it were summer and he were playing outdoors with friends digging in dirt the black would be easy to understand, but it's not. It's freezing outside and he had indoor recess at school. He did have pudding for lunch, but how could that have gotten under his nails? DS1 is a clean boy, no digging in his pudding with his hands, so how could this be? If anyone figures out how my son's nails could possibly get so gross, please fill me in. Until then, I am left to wonder...