ključati

1. (of a liquid) to boil
   Kad voda proključa, ubaci testeninu. — When the water boils, put the pasta in.
   Supa već ključa, smanji vatru. — The soup is already boiling, turn down the heat.
2. (of emotions, a situation) to seethe; to be in turmoil
   Ključao je od besa, ali je ćutao. — He was seething with rage but stayed silent.
   Posle utakmice ceo grad je ključao. — After the match the whole city was in turmoil.

Intransitive: to boil water (transitive) is prokuvati vodu, not ključati.