When you wash fleece, you take time to make sure that you don't accidentally felt the darned thing.
Deliberately shrinking your wooly knitted thing is entirely different, and so much fun!
Last year I made myself a pair of Felted Clogs. I felted them at my friend Ann's house since I don't have the top-loading washing machine needed for felting. She liked them so much I decided to make her a pair for her birthday this year.
Here's the "before" picture:
Just don't try this with a sweater!.