Explaining death to a 6 year old?

Friday • "In the end, we will protect only what we love, we will love only what we understand, and we will understand only what we are taught."
So, my sister's husband's grandma (my niece's great grandma) passed away a couple of days ago. My niece loved her "Mema," and doesn't understand what is happening or why Mema isn't coming back. My sister and her husband are trying to explain it to her as gently as possible, but this is the first time someone close to my niece has passed away. If you have lost someone close to your family, how did you tell your child and help them to understand?