expanding height with Divs

[tefnut]

Hi Im relatively new to the concept of Divs and CSS

Im trying to build a site with divs I have the standard 3 column stretchy width layout but have just found that I cant make it all stretch in height (and eventually scroll) when someone puts a load of stuff in the main content Div and make it become taller than the other 2 column.
This probably seems very silly not to know how to get around this but I could really do with some help. Id like it all to go the same height when stuff goes in the divs.

Thanks

[the tree]

Not silly at all, many developers have a lot of trouble with this. In fact, the idea of 3 fixed height collums has earned itself the nickname of "The Holy Grail of CSS layouts".
I don't have a sollution off the top of my head but I'm sure it can be done.