a lesson in emunas chachamim from "mayim she'lanu"

The gemara (Pesachim 42) tells a story of R' Masna who came to Papunya and taught them the halacha that matzah has to be made with "mayim she'lanu." The next day everyone came to his house to get water. He then had to explain that "mayim she'lanu" does not mean "our water,"i.e. to get the water from him, but rather water that rested outside its source so that it is cool.

Why do Chazal record this episode? Obviously there must be a better reason than giving us a chuckle at the expense of the simpletons in Pupunya?

R' Chaim Kanievsky explains b'shem R' Yisrael Salanter that Chazal are showing us what emunas Chachamim means. We've all baked matzah or seen matzah baked.  Little kids go on trips to the matzah bakery before Pesach.  Imagine a Rav comes into your local shul before Pesach, a visiting scholar in residence, and tells everyone a din in baking matzah that no one has heard of before.  Does anyone really think people will immediately change what they've always done?  Or would the reaction be, "Our parents baked matzah, our grandparents baked matzah, we've been doing this for years and no one has ever heard of such a crazy thing!"  The people of Pupunya must have baked matzah in years past before Rav Masna arrived on the scene.  Nonetheless, after they heard his derasha, they lined up next day for his water, as they misinterpreted "mayim she'lanu."  They were willing to accept on trust that Rav Masna knew what he was talking about even if it flew in the face of their past assumptions.