Hello! - I would like to fill in (the shape of the letters of) a word with another word (using repetition). What is the easiest way to do this in Illustrator (CS3)? Thanks!
There are many ways to do this and the one I would choose depends on the usage. Assuming you want to keep the text editable, one way to do is set the Opacity of the repeating word inside to 0 and stack it in front. Group all and in the Transparency panel click the box in front of Knockout Group until a checkmark appears.
This will make the repeating word inside see through as shown in the image by using the blue shape in the background.
Other ways of achieving this is using Opacity mask or Clipping Mask.