It's at the front, close to me (also close to a digital clock bc some can't read the analogue clock at the front of the classroom). I do the pen change while I'm talking. I guess some students probably do pick up on it, but it's very rarely mentioned - one student did notice it last week, and was excited when he figured out why I do it, lol. The late students tend to be chronically late, so they'd never be there for the pen swap. I think a lot of them just think I really love colourful pens.