Aさん: [something interesting] Bさん: なるほど。

While not unheard of in ordinary speech, 「なるほど」 is used with particular frequency by Japanese speaking nerds. 「なるほど」 is probably best translated as meaning something like "oh! now i get it" or "I see" (from my main man Jim Breen). Normal people use 「なるほど」 when someone has explained something to them and they have managed to understand it. This could be referred to as a "なるほど moment". They're usually pretty excited about what they've just understood and use it the way Archimedes is said to have used "Eureka!" — with an exclamation mark. Even so, its far more common to say something like 「そうなんだ！」 or 「今分かった！」 or even just 「あぁ〜！」. Note that for all the alternate phrases the exclamation mark persists. This is because regular people aren't often able to understand new things and are usually surprised and excited when they do.

Nerds, on the other hand are understanding new things all the time. A nerd is so used to having なるほど moments that when one comes along it's no biggie. Nerdy people who speak Japanese tend to use 「なるほど」 at a rate around 78% higher than average (source: my butt). They have a higher frequency of なるほど moments and because of this a nerd would usually not be moved to an excited state and would therefore not need an exclamation mark. Here are a few standard examples of usage where Aさん could be anyone and Bさん is a なるほど user.

• Aさん: We found the problem with the server. A malicious hacker broke in and deleted all files between 14K and 93K in size.
• Bさん: なるほど。

• Aさん: It looks like we cut your fugu wrong. You only have 3 hours left to live.
• Bさん: なるほど。

These 2 examples are pretty pedestrian. In each case a non-nerd would probably have responded differently, but its still clear why 「なるほど」 was used in each case. Now let's look at two more examples.

• Aさん: Before coming to work, I had breakfast and brushed my teeth.
• Bさん: なるほど。

• Aさん: I enjoy tennis.
• Bさん: なるほど。

OK. On the surface these don't appear to be なるほど moments, but they're might be some reason why they would be. It's not important that your reason for using 「なるほど」 be clear or even reasonable. What is important is that you appear to have an elevated understanding of what's going on around you, even if that's not necessarily true.

Usage is probably easiest to understand for people that often use "indeed" to respond to statements in English. The English "indeed" as a response can almost always be translated as 「なるほど」 in Japanese, but 「なるほど」doesn't have the automatic air of pretension that "indeed" does so the opposite isn't always true. In most cases, though, if you're a user of "indeed" in English, you'll probably enjoy using 「なるほど」.