Playing Ignorant out of Politeness

How often do you play ignorant out of politeness to the person that you are speaking to?

Case in point:

The other day, the water meter reader was at my house. You should know that when I moved into my house 5 years ago, the city installed a remote reader (known in the water meter reading industry as a Touchpad, I guess) so that they wouldn't need access to my basement every month. This is basically some RFID-like sensor mounted outside of my house with cabling that runs to the real water meter that is inside my house. I believe that the "touchpad" serves as the antenna to the actual transponder located on the meter itself.

There was a knock at my door, and when I answered, this middle-aged bearded man explained to me that my remote sensor was not working, and that he might need to get to my meter in the basement. We've got all kinds of crap piled up in the basement (ever see the TV show called "Clean Sweep"?), so I was really hoping that he would be able to resolve the issue externally.

After 15-20 minutes of him messing around outside my front door, he finally was able to get a reading (I think he replaced the external sensor thingy-majig). He knocked on the door again, and said that we were all set. Out of curiosity, I asked him what he had to do to fix it.

He went into a semi-technical lay explanation about how RFID works and that there's a cable running between the sensor outside and the meter inside. I patiently waited for his explanation to finish, while, meanwhile, my Raviolis inside were getting colder and colder. I think I threw some polite signs of interest, like "Oh, Really" and "No kidding?", but what I really was thinking was "Dude, I've written software for microcontrollers in assembly, and have also written simulators for those microcontrollers to run my software that I wrote in assembly. I think I can grasp the concept of a RFID-like water meter reader."

It then dawned on me that other people might do the same to me. I usually have no idea whether or not the person that I might be explaining something to has any domain knowledge, and depending on their reactions, I might tend to "dumb down" the conversation. What if this person is screaming inside their head "SHUT UP! I KNOW! SHUT UP!" ??? The last thing I want to do is waste their time or mine.

How often do you find yourself personally playing ignorant just to satisfy the ego of someone who is explaining something to you?