We saw last week that ci replaces preposition plus noun referring to place, or a + infinitive. Well it also replaces preposition + noun referring to things or ideas after certain verbs. The two most common verbs used with ci in this way are credere a (to believe in) and pensare a (to think about).
Credi agli UFO? - No, non ci credo. (= Non credo agli UFO.)
Do you believe in UFOs? - No I don't believe in them.
Pensi spesso al futuro? - Ma no, non ci penso mai! (= Non penso mai al futuro.)
Do you often think about the future? - No I never think about it!
There you have it - two more valuable uses of ci. Not bad for a two-letter word!