analogRead voltage fluctuation

I did a quick search but couldn’t find anything.
The problem we have is with using analogRead, the voltage we get fluctuates alot. It will rise to about 2/3 of the max value and then fall back down to zero for a second consistently for as long as we run it.
We need to read from each analog pin about 10 times a second.
Even with nothing connected to the pin, it will fluctuate in the same manner, but with somewhat lower values.

How and what is connected? :)

At this stage we were only reading from a resistor+led+9v batt circuit I crudely, but securely wired together for the time being while we are still coding the basic stuff. I just tagged off from the connection between the resistor & led to the analog pin. I'm probably doing something wrong with the electronics stuff, it's not so much my thing.. :-/

You need the led circuit and the arduino to have common ground. (Grounds connected together)

Do they?

Aha! No, they didn't. Problem solved, thanks for the speedy reply!

Grounds connected together

Can we have that as a banner headline?