Things that seem easy are often not.
Here's a puzzle:

Seems like: the voltage from the voltage divider lights the LED, at which point
the light from the LED changes the voltage at the voltage division point (more light
equals less resistance for a CdS cell) thereby dimming the LED, at which point
the voltage should go back up due to the now higher resistance on the CdS cell...
Then again, perhaps it's the opposite of an oscillator - where the feedback (via light) stabilizes the light...
Try it with CdS cells and also phototransistors!
Finally updated after ten years! First online 9/19/2013
fsw 09/17/2023