If you visit my www.ukfolkfestivals.co.uk site, on the country pages you will see some expanding/collapsing divs. You can copy the code (I got it from another site which gave it away free). Look for accordian.js. It uses two divs - accordianButton and accordianContent. It was originally written as a menu but is quite adaptable as you can see.
Then just apply css to get the expanded and collapsed sizes you want in the two states. the script applies two classes - "on" and "over" - which are the ones you will style.
Use a class, not an id if you want multiple divs of the same type. An id is supposed to apply to some unique item on the page eg a wrapper div or header div usually get an id as they are the only ones with that name. Also, ids can't start with a number.