Learn Spanish (Latin America): Esos rompecabezas son complicados.

Compound nouns formed by combining a verb and a plural noun, like rompecabezas, are invariable. This means they keep the exact same spelling for both singular and plural. You only know it is plural here because of the surrounding words: the plural demonstrative Esos, the plural verb son, and the plural adjective complicados.

How do we know the gender of a compound noun like rompecabezas?

Compound nouns made of a verb and a noun are almost always masculine, even if the noun part is feminine on its own (like la cabeza). That is why this sentence uses the masculine Esos and complicados instead of Esas or complicadas.

What are the two words that make up the compound noun rompecabezas?

It combines the verb rompe (breaks, from romper) and the plural noun cabezas (heads). Calling a puzzle a 'head-breaker' is a fun, literal description! Other common words follow this exact pattern, like el lavaplatos (the dishwasher).

Why use son instead of están with the adjective complicados?