Learn Hardcore Russian: Осторожно, кипяток очень горячий: держи кружку двумя руками. - Careful, boiling water is very hot: hold the mug with both hands.

Yes—Осторожно can function as a standalone warning meaning Be careful / Caution. The comma separates this warning from what follows (the reason/explanation). Similar to English Careful, it’s hot.

What exactly does кипяток mean? Is it just “hot water”?

Кипяток literally means boiling water (water at/near boiling point). In everyday speech it’s also used more loosely for very hot water (e.g., from a kettle or dispenser), even if it’s not actively boiling.

Why is it кипяток очень горячий (masculine горячий)?

Because кипяток is a masculine noun. Adjectives agree with the noun in gender/number/case:

кипяток (masc. singular, nominative) → горячий (masc. singular, nominative)

Isn’t кипяток already hot? Why say очень горячий?

It’s not “wrong”—it’s emphasis. Russian often reinforces warnings with extra intensity: very hot. In safety contexts this can sound natural, even if it feels redundant in English.

Why use очень here—can I say сильно горячий instead?

Очень горячий is the standard, neutral way to say very hot.
Сильно горячий is much less natural in this meaning; сильно more often modifies verbs (сильно обжёгся = burned oneself badly) or indicates intensity in other ways.

What does the colon (:) do in this sentence?

The colon introduces an explanation or instruction that follows from the warning:

Caution…: hold the mug with two hands.

You could also see commas or a dash in similar real-life writing, but a colon is common for “warning → instruction.”

Why is держи used—what form is it?

Держи is the imperative (command) form of держать (to hold/keep), addressed to one person informally (you singular). It’s telling someone what to do right now: Hold the mug…

How would it change if I’m speaking politely or to more than one person?

Use держите (imperative for formal you or plural you):

Осторожно, кипяток очень горячий: держите кружку двумя руками.

Why is кружку in that form?

Кружку is the accusative singular of кружка (mug). It’s the direct object of держи (hold what?):

держи (что?) кружку

Why is it двумя руками? What case is that?

Двумя руками is instrumental case, used for the means/tool by which an action is done: hold (it) with two hands.

двумя = instrumental of два (two)
руками = instrumental plural of рука (hand)

Could I also say обеими руками? Is there a difference from двумя руками?

Yes. Обеими руками = with both hands (emphasizes “both of your hands”).
Двумя руками = with two hands (more neutral/counting-focused).
In this context both are natural; обеими руками can sound slightly more idiomatic when talking about one person’s hands.

What’s the stress/pronunciation I should pay attention to?

Key stresses:

осторо́жно
кипято́к
о́чень
горя́чий
держи́
кру́жку
двумя́ рука́ми