I found this pattern in the book 'Kooky Crochet' by Linda Kopp:
2 liter purse by ~audreydc1983 on deviantART
This book is a must for a modern, quirky crocheter - it has some awesome patterns that you can twist to your own unique ends! (Such as an interesting hobo bag - I'll be doing a few versions in the near future).
So - buy it, or check it out from your local library. It's great!
All you need is a two-liter plastic bottle, a single hole punch, and a bit of yarn.
I made this for a daughter of a friend of mine for her birthday - she just turned 5!
Happy Birthday, Mika!