Rabbi Yehuda teaches that one must use מים שלנו (water that drawn before nightfall) for the preparing of מצה. Rav Masna taught this law to the people of Papuna and the next day they came to Rav Masna's house with their containers asking for Rav Masna's water. This is because they assumed the meaning of מים שלנו was "our water".
The lesson here is the amazing faith in the Rabbi's words that the people of Papuna displayed. They did not even question the requirement to use only the water of Rav Masna. If he said that was what was required, that's enough for them. How often do we discount the words of our Sages without even giving it the benefit of the doubt, let alone accept what we our told to do faithfully?