Learn Spanish (Spain): El agua está caliente.

We use está (from the verb estar) because the temperature of the water is a temporary state or condition. It is hot right now, but it could cool down later. We use ser (es) for permanent identities or essential characteristics, but estar for states that can change.

Wait, isn't agua a feminine word? Why does it use el?

Agua is indeed a feminine word! However, because it starts with a stressed "a" sound, saying la agua would blend into one long "a" sound. To make it easier to pronounce, Spanish uses el before it. It is still a feminine noun, which is why if we added an adjective like cold, it would be el agua fría.

Why doesn't caliente change to end in "-a" or "-o" to match the water?

Adjectives always have to match the noun they describe. However, adjectives that end in -e, like caliente (hot) or grande (big), are the same for both masculine and feminine nouns. They only change if the noun is plural (by adding an "s").

Why is there an accent mark on está?

The accent mark tells you to put the speaking stress on the final syllable (es-TA). It is also crucial because it tells the word apart from esta (without an accent), which means "this" (as in esta agua, this water). The accent changes both the pronunciation and the meaning!