Noah Smith offers this:

So how should we think about human capital? Here’s an analogy that I think works well. You agree that a chainsaw is capital, right? OK, now imagine a chainsaw that you graft permanently onto someone’s arm, like Bruce Campbell in the movie Evil Dead 2. It’s so thoroughly grafted on that you can’t remove it without making it permanently useless.

This chainsaw is very very much like human capital.

My prediction is that this metaphor will become increasingly apt, as implants, drugs, and genetic enhancements become a larger share of human capital. Today’s mouse capital is a preview.