(Strip Number 1166, Original Publication Date, 11 June 1987)
Panel 1: Having noticed that Baby Leah still has a pacifier, Lizzie asks Christopher why that is.
Panel 2: Rather than explain that she's teething, Chris just pops it out of her mouth.
Panel 3: This results in her screaming terribly and going all red in the face.
Panel 4: When he pops it back in and it soothes Leah, Elizabeth understands.
Summary: The reason for all of this is that Lizzie might not know what teething is and also it's a great sight gag.