Gordon found that three- to five-year-old children obey this restriction fastidiously. Showing the children a puppet, he first asked them, "Here is a monster who likes to eat mud. What do you call him?" He then gave them the answer, a mud-eater, to get them started. Children like to play along, and the more gruesome the meal, the more eagerly they fill in the blank, often to the dismay of their onlooking parents. The crucial parts came next. A "monster who likes to eat mice," the children said, was a mice-eater. But a "monster who likes to eat rats" was never called a rats-eater, only a rat-eater. (Even the children who made the error mouses in their spontaneous speech never called the puppet a mouses-eater).
I have not developed a mental mechanism that would drive the choice of mice-eater over mouse-eater, or of rat-eater over rats-eater (although anteater is, of course, a ready-made example that, on reflection, may come to mind).
