Since laws interest me, both personally and professionally, I try to acquire and maintain a smattering of familiarity with Canon Law. In pursuit to that goal, I follow Cathy Caridi's Canon Law Made Easy. It is, generally, an excellent source for interesting and insightful commentary on canonical questions.
But I believe she has made a remarkably unhelpful post on the topic of baptismal validity: "Why Is This Method of Baptism Illicit?"
In short, the interlocutor whose question is addressed in the post asked whether her Protestant baptism was invalid because the water did not flow across her: the Protestant minister "patted [her] on the top of the head" with "moistened" fingers.
Now, it's commonly accepted---or at least widely reported, see, e.g., William Fanning, Baptism, in The Catholic Encyclopedia (1907), available at http://www.newadvent.org/cathen/02258b.htm---that water must "flow" over the person's body in order for baptism to be effected.
But instead of addressing what it means for water to "flow," Ms. Caridi provides an (undeniably fascinating and of-itself-insightful) commentary on the liceity and history of baptism by aspersion. But that isn't really what the interlocutor wanted to know, or what she asked. What her question drove at is the different question of "how much flow is flow?" I would very much have liked to read Ms. Caridi's comments on the definition of "flowing water." Unfortunately, I didn't get to do so.