Sometimes I turn around from writing on the board and people on the right side of my class are laughing amongst themselves. This is when I am teaching. For the past year I've attributed this to my own (substantial) paranoia, until about an hour ago when after my lecture a student tapped me on the shoulder and said, "You have chalk on your butt."
I waited until most of my students were gone, then I craned my neck around and had a look at my butt. It had a big white line across one cheek. This line could only be from me sitting on the chalk ledge--the part of the blackboard where you store chalk.
At that moment, gazing at my own butt, I realized: I sometimes "rest" on the chalk ledge when I'm teaching. I hadn't consciously considered this before--it's just something I do without thinking while I teach. I feel tired, so I perch or lean on whatever is handy.
But when I perch on the chalk ledge: white line across butt.
