Nevin & Grace (N&G) buttress their metaphor with some good props. However, it is still not clear what momentum is analogous to. If momentum is a measure of strength, then the authors should say so and tell us how to calculate it. Furthermore, if “other” behavior can be introduced into the equation (and N&G's foray into the applied world suggests that it can), it is unclear when the masses are accrued and how much is accrued to each behavior.