Learn Spanish (Latin America): Tómate el caldo rápido, que luego se pone tibio. - Drink the broth quickly, because it gets lukewarm later.

In conversational Spanish, an unstressed que is often used right after a command or statement to casually explain the reason for it. It functions just like porque (because) but feels more natural and fluid in spoken explanations.

Why does the verb have a reflexive pronoun in Tómate? Wouldn't Toma work too?

Toma works perfectly, but adding the pronoun te makes it an aspectual use of the verb. With verbs of consumption (like comer or tomar), the pronoun adds a sense of completion or personal involvement, changing "drink the broth" to something closer to "drink up the broth" or "finish your broth."

Why use se pone to mean "it gets" instead of another verb of becoming like se vuelve?

The verb ponerse is used for temporary, usually reversible changes in physical state, appearance, or mood. Since the broth's temperature changing to lukewarm (tibio) is a temporary physical state, se pone is the correct choice here.

Can I use rápidamente instead of rápido?

Yes, you can use rápidamente, but in everyday Latin American Spanish, it is very common to use the adjective rápido as an adverb to mean "quickly." Using rápido here makes the command sound much more natural and conversational.