Skulls and bones
I have a thing about drawing skulls and bones. Not of any morbid fascination (I think), but because they can really be a challenge. The texture and colour of bones are interesting, and the ultimate challenge is to draw a skull first with graphite on white paper, then with white pencil on black paper. This is a massive challenge; drawing light instead of shadows. I have – as of now – no examples of the latter. And for bone structure; da Vinci is the boss.