(Strip Number 6171, Original Publication Date, 18 December 1988)
Panel 1: We find ourselves watching Elly and Lizzie wrap the Christmas gifts that'll be smashed to bits by baggage handlers because Elly wants a magic Christmas on the farm. As they do so, Lizzie asks if she can do a certain thing now only to be told to wait.
Panel 2: Said waited for thing is her putting her finger on the ribbon so Elly can tie the bow properly.
Panel 3: The strip proper begins with someone knocking on the door.
Panel 4: We find who was knocking when Lizzie asks Mike what he wants. What he wants is to head into the rec room.
Panel 5: When Lizzie tells him he can't because they're wrapping something, Mike gets all giddy and smiley and stupid and asks if it's for him.
Panel 6: Since Lizzie isn't very good at keeping secrets, she tells him "Not exactly...but sort of. I'm not going to say!" in the misguided and mistaken belief that she hasn't told him that yes, it is for him. Since everyone who isn't her realizes that she's just spoiled a huge surprise, he asks what it is.
Panel 7: When she vaguely alludes to as how it's something he wants, she thinks that he's still in the dark because she prefaced that comment with the phrase "I dunno." Since he isn't in the dark at all, he asks if it's so big and is a certain game from the computer store.
Panel 8: Lizzie says it's sort that big an'it might be a game an'maybe it coulda come from the computer store. Since she used maybes and couldas, she falsely believes herself to kept the secret safe.
Panel 9: Lizzie looks on in confusion when Mike thanks her profusely for unknowingly spoiling a surprise.
Panel 10: When she heads back in the room and tells Mommy that the hardest part of Christmas is keeping secrets, Elly is not impressed at all.
Summary: This is because of two things. The first thing is that Mike was able to wheedle the intel out of her no problem. The second is that she still to this day doesn't realize that she's terrible at keeping secrets. Why do you think no one ever told her about the Wedding For Heart?