For my first instructable, I'm going to show a trick that never gets old. Reusing Yarn is easy, and you only have to sacrifise your old ugly sweater. You do this with an awl or a sharp knife. And patience...
Step 1: Think About Stuff
When you choose what garment you're going to pull it apart, remember theese things:
1. Obviously, before you destroy your sweater you should check if someone else wants it.
2. The bigger the yarn is, the easier it is to pull apart.
3. It's harder to pull apart a sweater that has several yarns in it. You should choose a simple sweater with one colour, without patterns.
The sweater on the picture is your ideal! One-coloured, big yarn, and unloved!
Step 2: Cut It Up
Ok, so now you find all the seams and cut them open. This you do with an awl or a sharp knife. Avoid cutting anything else than the seams! (includes fingers). If you cut the yarn, it won't be the end of the world, but you'd later have to tie the yarn together so it's not in tiny pieces.
Step 3: And... PULL!
Now it's time to pull the yarn apart, rip it up! First you find the yarns ending, (if you can't find it, you can just make a cut in the yarn and begin pulling from there.) In the beginning of the ripping, you first have to pull each stich out from each other, so all stitches are "open" (like how picture number one shows). If you have cut the seams up properly, it should then work well to just pull the thread.
Pull the yarn with one hand and roll it between your other hand and elbow(like how the second picture shows). This can be a bit time consuming, so you'll need patience here...
Step 4: Tadah
And that's how you do it! When you're finished, the yarn will be curled like noodles, but don not fear! This can be fixed by steam-ironing the yarn carefully, and you'll have a great result!
Now enjoy your new yarn!