Explaining death to a 3 year old

This morning, our 17 year old rat terrier did not want to eat her breakfast. A home cooked meal of brown rice, ground beef and broccoli goodness. Casey has been spending more and more time sleeping in her crate, her favorite spot in the house--the warm kitchen. Our daily walks have been getting shorter and shorter. In human years, she is 119.

My daughter, a spirited 3 year old toddler that she is, wanted to chase her "sister" down for a hug. Casey wanted none of it. How do you explain to a 3 year old her "sister" is at the tail end of her golden years? Kayman loves her "sisters". We've talked about death in general terms, as both of her grandmothers died before she was born. We've chatted about dead snails and slugs we've seen on sidewalks after a rainstorm. We've never had the opportunity to discuss it as it is happening, until now.

As I am putting finishing touches on a new workbook for parents to explain pet loss to their kids, I realize I will get to be my own focus/test group by using it with my daughter Kayman.

Below are some of the things I will be discussing with Kayman.
-Write down what are some of Casey's favorite things to do, foods to eat, funny expressions of love.
-Draw pictures of Casey with her family.
-Write down "feelings" Kayman has around death.
-Explain what happens when an animal dies. They stop breathing. Their fur/coat is cold. They look like they are asleep but they are not going to wake up this time.
-Plan a good bye part for Casey.

I feel the gifts and the blessings of being able to share this with Kayman. And the gifts of this upcoming book with all of you.