That is the entire core of Russ Harris's work.

In desperation, you try to run away, but they chase you. The more you run, the faster they get.

Here is that core story, followed by a second essential one, and an explanation of why they are so useful. Imagine you are standing in a room. Suddenly, two huge, terrifying monsters burst through the door. They are Feeling Bad (a slimy, smelly beast) and Thinking Bad (a sharp-toothed, screeching monster).