What causes grainy photos?

[lyssa615]

I took around 100 photos the other day and only around 2 came out really grainy. What causes this? I used a P&S....
I would post 'em but I am at work now!

[2Shiny]

You were probably shooting on Auto mode, and the camera decided that it needed to bump the ISO speed in order to receive enough light and still have a relatively fast shutter speed. The higher you raise the ISO, the more grain you will notice on the pictures.
You can also create a grainy look to pictures if you do too much sharpening during post processing.

[goforphoto]

Bad exposure or iso being set too high is the most likely cause.