What to do on the daily commute

I'll admit it, I've become one of those annoying people sketching other people on the subway. Sometimes it's just feet or hands, and I feel, perhaps ostrich-like, less detectable. Other times it's angles of heads, fleeting impressions of faces or eyes. Occasionally someone will stay in the same position long enough to get a more developed sketch. No one yet has said "hey, you stop that!". One guy asked if he could take a cellphone picture of the sketch of himself. A woman asked if she could have the sketch. (Flustered, and not ready to tear the sheet out of the book, I said "no"-- but next time I will.)