What is a Columbarium?

After lunch Jack and I were walking from the cathedral to the van and we passed through the memorial courtyard. Jack asked me about the plaques on the wall and I explained that the plaques mark the recesses in the wall that contain the ashes of the person indicated on the plaque. Jack asked me about the unlabeled spaces and I told him that people can reserve space before they die and that the names are added when the spaces are filled.

Jack stopped to think.

He said "so, it's like calling Shotgun for when you die?"