"smart" dropdowns

Good evening, Does anyone have any grand ideas on how to use CSS (or any other
method) to have a dropdown box appear only if a certain other text field has
information put into it? I'd like the dropdowns to be invisible unless a user
puts a quantity into a particular text field. It won't be possible to use
layers -- they would have to be absolutely positioned (right?), and since there
are going to be many rows of these dropdowns, the page will be shifting height
possibly often. Thanks so much -- let me know if I need to explain this
better! Webley