[Zope] How to call functions in DTML?

Simple question from a stupid newbie. I'm used to writing inline code using PHP and I just want a random number to appear on a web page. Once I clear that hurdle, I want to use that number to reference different images (objects, right). Then I'll have a web page that displays random images from my image library. How do I call Python functions from within a web (aka DTML Document?) page in Zope? If you tell me to read the documentation I'll strangle you. :-) It's lacking. Thanks in advance, -Patrick Price