"the Krebs Cycle is like a merry-go-round in which two kids get on and two kids get off at every turn."
But
"The downside, however, is twofold. The first problem is that it's easy at times to think that the model is the reality. The goofier the metaphor, the easier it is to avoid this pitfall; I've never had a student yet who thought that the Krebs Cycle really was a merry-go-round (although I did have a student of mine start her essay on antibodies on the AP exam, "So, antibodies are trash tags..."). But with sophisticated, complex models, it's tempting to think that the model is, down to the level of details, what is happening in the real world."