In the last lesson, when we were talking about the plethora of uses for うち, I was only using hiragana for this word.

But, truth be told, it does have a kanji... sometimes:

内
うち
inside; within

Not "house," not "family," not "I."

This time, its meaning is "inside; within." Like this:

３つの内１つが正解です。
みっつ　の　うち　ひとつ　が　せいかい　です。
One of these three is the correct answer.
Literally: "three (things) + の + inside + one (thing) + が + correct answer + です."
Note: So, "Inside these 3 things, there is 1 correct answer."

In this particular example, the word 内（うち）is interchangeable with the other word for "in; inside:" 中（なか）. So we could also say...

(This time I'll write the kanji for the numbers, too.)

三つの中一つが正解です。
みっつ　の　なか　ひとつ　が　せいかい　です。
One of these three is the correct answer.
Literally: "three (things) + の + inside + one (thing) + が + correct answer + です."
Note: So, "Inside these 3 things, there is 1 correct answer."

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うち Goes to Grammar-Land

The "inside" うち we just saw can actually be used as a grammatical function meaning "while it's still."

I have one perfect sentence for memorizing this one:

熱いうちに食べてね。
あつい　うち　に　たべて　ね。
Eat it while it's still hot.
Literally: "hot + inside + に + eat + ね."
Note: In other words, "Eat it before it gets cold!"

We use this うちに kind of like the "while" in this sentence:

You should [do something] while it's A (because soon it will be B).
You should eat while it's hot (because soon it will be cold).

The cool thing about this is that the next time you're with someone getting some hot food, you can be like, Yo, eat it while it's still hot! 熱いうちに食べてね。

Other examples:

若いうちに勉強しておけばよかった。
わかい　うちに　べんきょうして　おけばよかった。
I wish I'd studied when I was younger. // I should have studied while I was still young.
Literally: "young + うちに + study + if prepared + was good."
Note: You should just memorize ～ ておけばよかった as "I should have ～ (ahead of time)."

If this sentence is making your brain melt, you might want to take a look atLesson #39, where we tear this grammar apart.

One more:

ケンタがいないうちに食べちゃおう。
ケンタ　が　いない　うちに　たべちゃおう。
I think I'll eat this before Kenta gets here! // Let's eat this before Kenta gets here!
Literally: "Kenta + が + is not (here) + うちに + let's eat (everything)."
Note: This could be one person talking to himself or herself, or it could be two people. Maybe they're talking about a cake. When Kenta comes, they'll have to split it three ways! But if they eat it "inside" the time he's not there, they can each get half. Also, ～ ちゃおう is an abbreviation of ～ てしまおう （i.e. ～ てしまう in volitional form).

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How does this grammar work exactly?

Basically, this specific usage of うちに will only attach to the following:

NOUN + の + うちに
な-ADJECTIVE + な + うちに
い-ADJECTIVE + うちに
PLAIN PRESENT POSITIVE VERB + うちに
PLAIN PRESENT NEGATIVE VERB + うちに

Agh! Grammar formulas. <(ToT)>
Make it stop. Make it stop.

How about I simplify instead?

Here's our Word Toolbox:
独身（どくしん / "single; not married"）
　＝　NOUN (We must add の ！// Also, some dictionaries say things like "bachelorhood" for 独身 in a feeble attempt to match the Japanese noun to an English noun, instead of the adjective "single.")
健康（けんこう / "healthy"）
　＝　な-ADJECTIVE (We must add な ！)
　　　*健康 can also be a noun,
　　　but we're going to be using
　　　it like a な-adjective today.
若い（わかい / "young"）
　＝　い-ADJECTIVE
いる （"to be [for living things]"）
　＝　PLAIN PRESENT POSITIVE VERB
いない（ "to not be [for living things]"）
　＝　PLAIN PRESENT NEGATIVE VERB

That means these words will look like this:
1) 　独身　 の うちに
2) 　健康　 な うちに
3) 　若い　 うちに
4) 　いる　 うちに
5) 　いない　 うちに

1) while one is still single
2) while one is still healthy
3) while one is still young
4) while one is still here/present
5) while one is still not here/present

Full Examples

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1)

　独身　 の うちに遊びまくる！
どくしん　の　うちに　あそびまくる！
I'm gonna party it up while I'm still single!
Literally: "single + の + うちに + play like crazy!"
Note: 遊びまくる means something like "play with reckless abandon." You can add ～ まくる to ます-stems to mean "VERB like crazy." For example, 食べまくる（たべまくる / to eat like crazy).

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2)

　健康　 な うちに たくさん旅行しよう。
けんこう　な　うちに　たくさん　りょこう　しよう。
Let's travel a lot while we're still healthy. // I think I'll travel a lot while I'm still healthy.
Literally: "healthy + な + うちに + a lot + traveling + let's do."

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3)

　若い　 うちに 結婚したい。
わかい　うちに　けっこん　したい。
I want to get married while I'm still young.
Literally: "young + うちに + marriage/wedding + want to do."

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4)

先生が　いる　 うちに 聞いておこう。
せんせい　が　いる　うちに　きいておこう。
Let's ask the teacher while she's (still) here. // (I think) I'll ask the teacher while she's (still) here.
Literally: "teacher + が + is + うちに + ask + let's do (in advance)."

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5)

ママが　いない　 うちに遊びに行こう。
ママ　が　いない　うちに　あそびに　いこう。
Let's go out and play before Mom gets home. // (I think) I'll go out and play before Mom gets home.
Literally: "mama + が + is not + うちに + playing + に + let's go."
Note: ママ kind of sounds like "mommy," so some of you probably will want to avoid using it.

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The End!

Have fun getting all of that "inside" your head.
(Also, don't hate me for making lame jokes. I'm in a fragile mental state, having just written a big long Japanese lesson.)