Security images

hey guys.

you know when you register for a website and there is a security image?

its an image with letters or numbers in it, usually squigly and distorted.

the user has to enter the characters in the image into a textbox to verify his humanity, and that he isnt a bot

this is the kinda thing i mean:


how is that done?

i thought it would be an iframe with the image and a hidden field, but surely a bot could read a hidden field?

ive searched the forums, but couldnt find what i was looking for.

so, any ideas as to how its done?