A simple rule to check whether a verb is transitive or intransitive (requires a direct object or not):
Ask the question What or whom
I opened ... (what did I open?) I opened the window. Window is the direct object. "I opened" is not a complete sentence, the object is missing!
Therefore to open is transitive verb which requires an object
I called (whom?) I called him
to call is transitive verb
to rain, to snow: it rained, it is snowing : These are complete sentences and the verb does not require an object (intransitive verbs). likewise to sleep, to wake up etc. are intransitive verbs.