I'm not completely new to Linux, I learned on Ubuntu (so that may or may not still qualify me as n00b) but I've never had to use any repositories other than the ones that were provided with my particular distro (aka Ubuntu 9.10).

If I were to download packages from other distros, would they...give...my BackTrack...cooties or something? Should I not add other repositories to my sources.list? I've read the FAQ, and so I know that I SHOULDN'T add non-bt repositories, but now I'm just curious about what WOULD happen...and more importantly WHY would it happen. My very basic understanding of Linux environments is that they're all based on the same basic structure (which is the same structure that proprietary OSes are built from), but I've never really understood why one group of apps from one distribution of Linux wouldn't work with another.

Like...if I had a tree (UNIX) and then I cut a branch off of that tree (Linux) and further cut that branch into segments (distros) since they all came from the same branch of the same tree, it doesn't make sense to me that if I were to use one piece of wood to make a house (Ubuntu), and another piece of wood to make a bird house (BackTrack) why they would react any differently if I were to light them on fire (computers on fire = highly toxic).

If anyone can answer my original question(s) (***OC) in terms of the ridiculous tree analogy above, you'll get free cookies.
...In the sense that you will have another cookie on your computer that entails you answering yet another stupid question to a noob on the internet.