Slimes, in the old ZZT days were used for only one purpose. To entirely fill the board in breakables, trapping the player from getting out without bullets. Later on, a neat new thing that could be done with slimes was figured out. From what Dragonlord has heard, it was invented by Viovis. He could be wrong. But, the method was a very good discovery. So if you're tired of making backgrounds for your games, fear no more!

Before you do any of these, you need a type-changer. Here, make an object and paste this coding:
@mixer
#touch
#end
:touch
#if not any slime #die
#lock
#cycle 1
#change normal invisible
#change water normal
#change solid water
#change breakable solid
#change invisible breakable
/i
#touch

It basically changes every type except it keeps the fakes fake. Now that you've got that done...

Using slimes to make single colored backgrounds
This is the easiest way of filling in a background. To do this, put the color changer somewhere on a board. Then take slimes and put them in random places, scattered about the board (except for the corners). The more you put, the smaller the blobs of terrain will be. Make sure you have the slimes on the fastest speed. I recommend doing this on the title screen so you can hit E, T and export the board. Here's an example of what came out for some dark cyan slimes: (Click to enlarge)

This is before to after. You should see something like this when you do your slime mix.

Using slimes to make two or more colored backgrounds
This is basically the same as above except you use two (or more) differently colored slimes. Spread them out around the board like before. It's just like the one above.

Black slimes - Misunderstood Shapeshifters
People don't know how powerful black slimes can be. Black slimes will fill in any colored fake with a random terrain (using the object). On the downside, it makes all the empties white but all you have to do is insert #change white solid empty, #change white normal empty, #change white breakable empty, and #change white water empty at the end of the object. Here's a little something I did with a single black slime:

As you may have noticed, his head got demented during the process. ;) I also added in a #change invisible solid command at the end because there were a lot of left over invisibles. So there.