We have a gem and we want to share it with you. It's a little slip of the tongue that Rosie, one of the girls in the NPS series, makes while being introduced to a handsome new sports instructor:
 

Ay, a mí me encanta el deporte y más si el "teacher" está así de bueno.

Oh, I love sports and even more if the teacher is so good-looking.

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Rosie's subconscious betrayed her for a moment there, because that's apparently not what she wanted to say, as she immediately corrects her blunder:

Ah, ay, digo, digo si es tan bueno.

Uh, oh, I mean, I mean if he's so good.

The difference between estar bueno (to be good-looking*) vs ser bueno (to be good) is the classic example used to explain the proper way to combine the verbs estar and ser (both meaning "to be") with adjectives, and to understand the sometimes not-so-subtle difference in meaning that results from it: if you use ser, the adjective is a fundamental characteristic of the person or thing you are describing, whereas if you use estar, it's a description of a mood or appearance, something less intrinsic or something not permanent. Having the chance to learn this rule with a pun is priceless, don't you think?
 
There are many interesting examples of adjectives that change meaning when they are combined with the Spanish verbs ser and estar to describe people. For example, the adjective frío, which means "cold."
 
You can use this adjective with the verb ser to describe a fundamental characteristic of a person or group of persons:
 

Lo siento. Pero acá la gente es fría y distante, es una... -¡Mentira!

Sorry. But here the people are cold and distant, it's a... -Lie!

Caption 73, Yago - 10 Enfrentamientos

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But if you say that someone está frío, that can only mean that the person('s body) is actually cold. Here is a grim example:
 

Está en la cama, muerto. Está frío y azul.
He's on the bed, dead. He’s cold and blue.

 That's why, in fact, the combination of the verb estar with the adjective frío is much more commonly used to describe objects, concepts, and beings regarded as inanimate: la noche está fría (the night is cold), la champaña está fría (the champagne is cold), etc. But careful: that doesn't mean that you can't use ser + an adjective to describe such things. You can, especially with concepts and abstract ideas. For example:
 

...si la temperatura exterior es más fría que la interior

...or if the temperature outside is colder than the inside [temperature]

Captions 58-59, Tecnópolis - El Coronil

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En Buenos Aires las noches están frías.
In Buenos Aires nights are cold.

 
Yet that doesn't mean that you can't say en Buenos Aires las noches están frías. It's just definitely less common and actually incorrect if what you mean is that all nights in Buenos Aires are generally cold. So, if you ever find or hear such an assertion using the verb estar instead of ser, it would probably be accompanied by certain implicit or explicit clues that would tell you that the adjective frías (cold) is being used to describe a temporary situation. For example:

En Buenos Aires las noches están frías, por ahora.
In Buenos Aires nights are cold, for now.
 
No salgas, está frío afuera.
Don't go out, it's cold outside.

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 So, you may be wondering: how do I say in Spanish that someone is cold, meaning that the person feels cold? Well, you have to use a different verb instead: tener (to have). Have you ever heard a Spanish native speaker say "I have cold" by mistake? That's why.

...y yo nada más tengo frío y hambre y no sé qué hacer.

...and I'm just cold and I'm hungry and I don't know what to do.

Caption 23, Yago - 6 Mentiras

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So, unless you are a zombie or another kind of undead creature, don't ever say estoy frío.

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*Just so you know, the adjective bueno in estar bueno is actually closer to "yummy" or "hot" than to "good-looking."