I’m frustrated, so that makes me wrinkle my forehead, which makes me frustrated that my forehead is wrinkled, which makes me wrinkle it more, which makes me even more frustrated, which makes it even more wrinkled, which makes…
If you clip your fingernails (I’m giving you the benefit of the doubt here—it better not have been toenails) on the bus, expect all of your fellow passengers to wear expressions like this.