If you measure an optocoupler's output while driving its input directly from a 9-volt battery, you get a 2-leg transistor.
laugh! ha ha ha!*
You can still use it as a temperature sensor or maybe even as a radiation sensor.
"Merry fucking Christmas!"**
*) I HATE those movies where they insert laughter in the background whenever some idiotic underpaid actor recites some funny line. It's an insult to my sense of humor. Is it like, am I too stupid to figure out for myself when to laugh and when to go take a shit? Good comedies never insert that stupid laughter. Only mediocre ones that target mediocre people need to.
**) a famous quote by Mr. Garrison.