Somewhere in their first year or so of studying Japanese, most students (myself included) will learn the phrase:

なるほど
naruhodo
I see.

This is "I see" in the sense of "Ah, now I understand."

It's something you might say to your Japanese teacher after they finish explaining something to you.

However, the thing they're explaining should be interesting and/or difficult to understand. Kind of like saying, "Oh, now I see."

This is different than saying something like:

分かりました
wakarimashita
I got it. // Noted.
(Literally: "understood.")

More casually:

分かった
wakatta
I got it. // Noted.
(Literally: "understood.")

This is less like, "Oh, now I see," and more like, "OK, that makes sense," or "OK, (keep teaching; you haven't lost me yet)."

Let's say that each "Oh, now I get it" type of phrase has two variables:

1) 面白さ
omoshirosa
"Level of Interesting"
(Note: The connotation of the adjective 面白い [omoshiroi], or in this case the noun 面白さ [omoshirosa] is actually quite a bit more positive than the English word "interesting." This is why you'll sometimes see it translated to "fun.")

2) 難易度
nanido
"Level of Difficulty"

So なるほど has the nuance that the thing I've learned has both "High-Difficulty" and "High-Interesting."

分かった has "Low-Difficulty" and "Low-Interesting."

(Click any image in this lesson to enlarge.)

This is important!

Because a lot of beginner students will say なるほど (naruhodo) when they learn something with (A) "Low Difficulty" and (B) "Low Interesting..." which can make you sound (A) mentally handicapped and (B) super weird.

But what if I want to show that something newly learned is interesting, even though it's not particularly difficult.

For example, let's say you told me...

A:
一日の心拍数は10万回以上もあるんだって。
ichinichi no shinpakusuu wa juu man kai ijou mo aru n datte.
I read that the human heart beats over 100,000 times a day!
(Literally: "one day + の + heart beat number + は + 100,000 times + over + も + there are + ん + they say.")

(Note: There's so much fun grammar here, but we don't really have time for it in this lesson. This ～だって [~datte] just means that the speaker heard or read this fact somewhere... that's why we have those "colloquial quotation marks" in the form of って.)

Now, that's really interesting (I think), but it's not hard to understand. It's just a fact, so it doesn't require any kind of mental calculation--you just accept or you don't.

Let's say you're not a total liar, so I do believe you, then I might respond with...

B:
へぇ～、知らなかった！
hee~ shiranakatta
Really? I didn't know that.
(Literally: "[sound marking interest] + didn't know.")

I could also say:

へぇ～、初めて知った！
hajimete shitta
Really? I didn't know that.
(Literally: "[sound marking interest] + for the first time + knew.")

"High Interesting" + "Low Difficulty."

******Sudden Digression******

I should say a couple of things about this へぇ～.

First, I had no idea how I should write this, and I had to do a bunch of research to figure out how Japanese people write it.

Second, this sound that Japanese people make isn't really a word--it doesn't mean anything.

And yet, it also means SO much.

When a Japanese person learns something new and interesting, they will show that it is new and interesting (i.e. that they are interested and learning new, fun stuff) by saying へぇ～.

I tried to find some examples on YouTube... but after failing at that for 15 minutes, I gave up. (I did, however, find this attractive punching blowup doll.)

Just teach a Japanese person something interesting. They WILL say へぇ～.

There are lots of sounds like this in Japanese. They can be pretty hard to master, because books are really bad at explaining them. If you keep your ears open, though, and try to imitate the speech patterns of native speakers, you'll start picking up these response-sounds, as well.

I know that for me, personally, they've started slipping out in English conversations, which has earned me more than a few strange looks.

******End Digression******

To give a completely different example, what if we were told some new information, we had to think about it for a second, and then we wanted to say, "Maybe you're right..."

Any ideas?

Let's look at an example...

A:
月曜日は本田はいつも機嫌悪いからね。
getsuyoubi wa honda wa itsumo kigen warui kara ne.
Honda is always in a bad mood on Mondays.
(Literally: "Monday + は + Honda + は + always + disposition + bad + because + ね.")

(Note #1: Note that this is the ほ [ho] sound for Honda: ほんだ. Not the English pronunciation of the car company Honda: はんだ. In this case, it's just some dude's last name.)
(Note #2: The nuance of this からね [kara ne] is that person A is explaining why Honda is in such a bad mood today.)

B:
言われてみれば確かにそうかも
iwarete mireba tashika ni sou kamo.
Now that you mention it, I suppose that's true.
(Literally: "was said [to me] + if look + certainly + is so + maybe.")

(Note: Another translation for this might be something like, "Now that I think about it, [maybe] you're right.")

This phrase "言われてみれば…[something]" is really useful for "Now that you mention it... [something]" phrases.

It has the nuance of "High Interesting" and "Somewhat Low Difficulty:"

We can also say "言われてみると...[something]."

Example:

A:
金曜日は本田はいつも機嫌いいからね。
kinyoubi wa honda wa itsumo kigen ii kara ne.
Honda is always in a good mood on Fridays.
(Literally: "Friday+ は + Honda + は + always + disposition + good + because + ね.")

By the way, we're ending this phrase with ～そうかも (~sou kamo), "maybe that is so."

This かも is actually an abbreviation of ～かもしれない／～かもしれません... which is a grammatical construction equivalent to saying "maybe" in English (we'll look at it some other time).

Actually, though, we can put all kinds of things after 言われてみれば and 言われてみると. I just thought that ～そうかも（しれない） was really common. So common, in fact, that these exact phrases have tens of thousands of results in Google:

If someone pointed out that two things are similar, you might say:

言われてみれば確かに似てる。
iwarete mireba tashika ni niteru.
Now that you mention it, they are similar.

B:
言われてみると確かにそうかも
iwarete miru to tashika ni sou kamo
Now that you mention it, he does seem to always be in a good mood on Fridays.
(Literally: "was said [to me] + look and + certainly + is so + maybe.")

Or if you're just realizing that something is true, you can say:

言われてみればそうだね。
iwarete mireba sou da ne.
Now that you mention it, that's true.

And I think the most common one is to chop off that そうだね and just say:

言われてみれば
iwarete mireba
Now that you mention it, (that's true).

Although 言われてみると seems to have more results online, I definitely hear 言われてみれば more often.

...but I'm not sure if that's because it's more common in spoken Japanese or just because Rei uses it a lot.

Bonus Phrases

面白さが全然理解できない。
おもしろさ　が　ぜんぜん　りかい　できない。
I don't understand what's funny [interesting] about this at all.
Note: The connotation of the adjective 面白い [omoshiroi], or in this case the noun 面白さ [omoshirosa] is actually quite a bit more positive than the English word "interesting." This is why you'll sometimes see it translated to "fun."

面白いアニメ知ってる？
おもしろい　アニメ　しってる？
Do you know any good [interesting] anime?

難易度高すぎ。
なんいど　たかすぎ。
This is too difficult. // The difficulty level is too high.

お母さんも来るかもしれない。
おかあさん　も　くる　かもしれない。
My mom might come, too.

明日は台風かもしれません。
あした　は　たいふう　かもしれません。
There might be a typhoon tomorrow.

そうかもね。
そう　かも　ね。
Yeah, maybe.