(Strip Number 1347, Original Publication Date, 20 September 1988)
Panel 1: We find Lizzie and John in mid-hug as she tells him that she can hear his heart beat.
Panel 2: She sets up the punchline and arc by asking what'd happen if it stopped. Since Daddy is a knuckle-dragging imbecile who doesn't 'get' what frightens kids, he smiles broadly as he acts way out of character and tells his eight year old daughter that he'd 'stop'.
Panel 3: Since John isn't insulting someone or making a joke about a troubling issue to preserve her
Panel 4: El Sensitivo shows us where Mike got having his head up his ass from when he's astonished by Lizzie's plea about wanting his heart to beat forever and ever. This is because he doesn't realize that she might have thought that he'd prove his point by having a fatal heart attack on the spot.
Summary: As I said, this does not lead to an examination of Elizabeth's abandonment issues or doing something stupid and irrelevant like "reassuring a child" because children are 'resilient'. This actually leads to the yuppie scum dentist angsting about having to have a checkup because he doesn't want to eat his spinach.